Dear Mr. Waite,
I appreciate the concerns expressed in the critique of our paper, and
have attempted to address each of these, as noted in the detailed
section below as well as in the revised manuscript. However some of
the criticism concerned not the content but the style of the
presentation for which I take full responsibility, and for which I
make no apology. An even more difficult issue for me to respond to
were the legal concerns raised by referee A. Both of these issues are
peripheral to the scientific discussion, and I am sure that with some
help from the referees, we can come to an amicable compromise. I hope
that my defense of these issues will not be taken as recalcitrance,
but instead as an important apologetic of the role scientists play in
shaping society. Thus I have separated the my response into two sections:
the first concerning the non-scientific questions, for which I would
greatly appreciate an editorial opinion, and the second half the
scientific issues.
General Non-Scientific Concerns
Because these are ``meta-issues'', not directly relating to the science
of the paper but to the impact of this important journal on society,
I would greatly appreciate the inclusion of the GRL editor into this
debate. I will gladly accede to the wishes of the editor, but only
after he has read my defense.
1) Both referees remarked that the tone is ``sensational''. I do not
refute that observation, but I do refute the implication that it is
undesirable, at least, in the sense defined by Referee A. Let me
illustrate with two examples.
a) The twin GRL articles published 10 years ago on ``cometesimals''
were, at the least, as ``sensational'' as this manuscript. Did it
destroy GRL's reputation? Did it inhibit scientific debate? On the
contrary, it no doubt enhanced GRL's circulation and promoted many
lively debates on a subject that is still ongoing. (See the July issue
of Physics Today. When was the last time Space Physics made it into
the trade journal of US physicists?) Was the science any worse for
being sensational? Probably not, though it prompted much
soul-searching as to the way observational science is conducted and
the characteristics of marginal science.
b) The claims and the press surrounding the Mars meteorite would
probably qualify as sensational. Yet we are told that this is a
necessary expedient to get the attention of the American public. We
are told that this ``origins'' initiative alone is responsible for an
extra \$200M in the planetary program. We are told that ``relevance''
means writing for the ``tabloids'', making our science understandable
and applicable. That in the past scientists were content with their
ivory towers but today we must justify to an increasingly skeptical
public the importance of our work. Let us then not wink with one eye
at sensationalism that benefits us while turning an evil eye on
sensationalism that doesn't.
Let me try to rephrase the accusation of sensationalism. We should all
avoid self-serving publicity which richly deserves the epithet
``sensationalism'' and desire to serve selflessly the society which
supports us, providing that society with relevant information that can
benefit us all. If in my desire for relevance I have appeared
``sensational'' then I stand corrected and rebuked. But if my accusers
can only point to my use of concrete examples and jargonless speech as
examples of ``sensationalism'' then I fail to see evidence of
self-service.
2) Referee A would like me to use ``energetic'' or
``outer radiation belt electrons''
to describe ``killer electrons'', viewing the latter as an
unscientific definition, and an example of sensationalism. Let me
begin my defence by saying that I did not invent this phrase and originally
felt the same way about the use of it, but I have slowly come to
change my views. Obviously, it can be changed without affecting the
science of the paper. But there are good reasons to use such a
descriptive adjective:
(a) I would beg the referee to consider such scientific terms as
``whistlers'' or ``lion-roars'', which can be both very graphic as
well as scientific. There is an early space physics volume with a cartoon
of the anthropomorphisms used in space physics in the early 60's,
including ``Carpenter's Knee'' and ``Ness' long tail''. Think
carefully over the use of ``radiation'' in ``radiation belts'';
sunlight is radiating, but this is not the connotation of this word,
rather ``radioactive'' or ``ionizing radiation'' comes to mind, which is precisely why the word
has stuck. The question is not whether the term is scientific, but
whether it is necessarily descriptive, e.g., does it fill a void by
capturing the essence of a category that otherwise receives ambiguous and
inconsistent long descriptors?
(b) Other titles:
``Energetic'' is far too vague an adjective. It means 1 eV to a
ionosphere person, 100 eV to a plasmasphere person, 100 keV to a ring
current specialist, 100 MeV to SEP person, and so on.
``Outer radiation belt electrons'' is both cumbersome and inaccurate,
since it specifies a place rather than an identifier, or more
precisely, it makes an identifier appear to be a location, since the
outer radiation belt is defined by the presence of MeV electrons (note
the circularity of the definition!). What then does one call 10 keV
electrons also found in the outer zone? What about low-altitude
measurements of electrons made with SAMPEX? What if, as we argue in
the paper, these electrons originate outside of the radiation belts,
should they change their name as they diffuse into this location? Is
there really a difference between ``outer'' and ``inner'' electrons,
between a 1 MeV electron found at L=5 and found at L=3? Are there
``outer radiation belt protons'' as well (can one divide the protons
into an inner/outer belt)? In my mind, this phrase has led to more
confusion concerning the definition of the radiation belts than it has
clarified the population it attempts to distinguish.
``MeV electrons'' is more specific, but it still spans 3 decades of
energy, when only the lowest decade is meant. Nor is it in SI units,
which violates AGU standards. Nor is it easy to pronounce, usually
``em-ee-vee'' is used, with all the cachet of ``eff-tee-ee'' or
``gee-see-em'', which is to say, jargon to the uninitiated. (This
raises the question, why are SI units pronounced fully, nPa =
``nano-Pascals'', whereas these non-SI units remain tri-syllabic
jargon?)
``Relativistic electrons'' are also awkward, first, because it
sets only a lower energy limit, and second, because it emphasizes a property
of the population that seems irrelevant to its measurement or societal
impact. Thus it seems we really do have a problem naming the
population we are studying.
(c) In picking a more descriptive name, one should aim for accuracy if
not in denotation, at least in connotation. Rather than attempting a
purely denotative name (such as auroral kilometric radiation) that
rapidly collapses into tri-syllabic jargon, I have chosen to use a
descriptor coined, I believe, by Dan Baker, and used by him for at
least 2 years at AGU talks for this exact population. The connotation
of this word ``killer'' is that the electrons are dangerous to humans
and life, and perhaps secondarily, to complicated human
machinery. From a non-scientific and societal point of view, this is the
most important property of this population and might even be called a
defining characteristic. If we have our ``radiation belts'', is it
such a long stretch to have the ``killer electrons'' that inhabit it?
3) Referee A mentions that since insurers paid $>$\$100M for Telstar 401,
it has an impact on what should be published in the scientific
literature. ``With this magnitude of liability, one should be careful
about reporting or publishing statements about the Telstar satellite
that cannot be supported by evidence.'' Naturally, as scientists, we
should always be careful about making statements that cannot be
supported by evidence. At the outset, I acknowlege that I should have
been more careful when attributing this failure to a DDD, though I do
believe there is much evidence in support of this working hypothesis.
The fact that the HST mirror cost NASA in excess of \$500M did not
hinder scientists from publishing accounts of the error or on
speculating on the reasons for such an error, nor should it hinder us
from attempting to understand the failure of Telstar 401.
One difference, perhaps, between the HST and Telstar 401, is that NASA
is self-insuring whereas private insurers covered the cost of the
satellite. Therefore I have called the world's leading insurer of
telecommunications satellites, Generali of Trieste, Italy. I spoke
with a deputy of Mr. Paolo Albanese, a Mr. Citrone (sp?) in the Space
Department, about the impact of scientific research on insurers and
the liability I might incur. He said that his company was not directly
involved in the Telstar 401 claim, and he expressed surprise that I
should incur any liability. He said that he couldn't speak for the
American legal situation, but it wasn't common in Europe. He then
asked me if I was offering my services to the company and remarked that several universities and private
corporations had already contacted them about doing risk assessment
for the problem of ``radioactive electrons''. He seemed well aware of
the space weather implications and seemed to think I had much company
in this research.
The conclusions I drew from this phone call, therefore, are that GRL has
little to fear about liability in publishing these results, that they are
already well known to the insurance companies. If anything, the
publication of scientific studies might undercut the market for
private risk assessments and reduce their value. Before GRL supports a
gag rule based on fear of law suits, they would do well to assess the
possible other motives for such a decision.
Specific Scientific Questions
Referee A:
0) Referee A argues that we have been guilty of factual inaccuracy in
too many places to recount. This is unfortunate, because our desire is
to publish a factually correct report. I will address all those mentioned
in the referee report, and beg the referee to make a complete list, if
possible, available to us. Our e-mail is: r*sheldon@bu.edu, and if the
GRL editor would forward it, the authorship would remain anonymous.
1) Point one is mainly about style, which I have addressed above. The
last sentence concerns the Telstar 401, which is repeated in point 2 below.
2) The referee asserts several times that I have ``NO EVIDENCE'' that
MeV electrons were responsible for the Telstar 401 failure. This is not
strictly true, though the referee may still disagree with me about which
evidence is admissible.
a) We have the data from Geosynchronous spacecraft, from POLAR, and
from low-altitude spacecraft that the MeV electron environment changed
dramatically on January 10. We know from SCATHA published results that
this type of radiation environment leads to spacecraft anomalies. We
have on record that the ``red limit'' level of MeV electrons that the
SEC monitors in sending out their environment warnings was
exceeded. We even have data from Telstar 40?, which was equipped with
voltage monitors, showing anomalous behavior on 10 Jan, as reported at
the Spring AGU.
b) I have had numerous conversations with scientists and commercial
spacecraft operators that informally confirmed to me the connection
between the space environment and the spacecraft anomalies. This
admittedly is anecdotal, but nonetheless revealing. The spacecraft
operators said that they were prepared for such outages by having
transponders rented on multiple satellites because it was well-known
that the satellites built in East Windsor were susceptible to this
sort of problem.
c) The scientists most familiar with the problem of DDD are those
working the DoD, since the Air Force has been carrying out research on
this possible threat to national defence even before SCATHA and CRRES.
Unfortunately these reports are not well circulated, so I can only
refer to the published SCATHA material. Evidently the location of a
DDD can be in the dielectric material shielding the cables of a
spacecraft, where it induces large voltages across control lines. One
possible but expensive cure is to protect with diodes every input
leading in or out of a piece of hardware. This DoD solution was most
likely not implemented on the Telstar 401. Even more revealing, the
official AT\&T failure report, as presented by Dr Lou Lanzerotti at
the Spring AGU, denied all space weather influence and instead listed
3 possible mechanisms, point 3 being ``failure of the Teflon
insulation''. This denial of space weather influence at this meeting
was met with a murmuring wave of disbelief from the audience, who, no
doubt, had vested interests in space weather.
The June 1 Space News carried a report with the title, ``Faulty
Materials Blamed in Failure of Telstar 401''. Quoting extensively from
article we read:
``The investigative report, which has not yet been made public, cited
a manufacturing error or faulty materials as the cause of an
electrical short that permanently shut down Telstar 401 Jan. 11,
according to an industry source...After the satellite failed,
speculation arose that a solar disturbance could have caused the
electrical short, but an industry source said that is not the case.
Instead, the satellite is believed to have suffered a short circuit in
its power supply unit because of a hardware failure, the source
said. Three possible causes were identified: a bus bar made of tin and
used to distribute power throughout the satellite frayed; a meter
shunt, a device often used to measure current traveling through a
circuit, was over-tightened; or Teflon insulation used to cover the
satellite's wires somehow triggered a shock. A satellite engineer at a
competing company said it is not uncommon for Teflon to be used as
insulation in satellites. Three insurance claims totaling \$150
million have been paid for the dead satellite.''
I cannot refute that AT\&T has denied a space weather influence, though
I leave it to the referee to decide how AT\&T knew that space weather
had no impact on its failure mode \#3. I will not speculate on the
motives behind such categorical denials, but at the least I hope I
have answered the referee's claim that I had ``NO EVIDENCE'' for
making the hypothesis.
3) The referee does not argue with the claim that we have convincingly
shown that the Earth's cusp can trap particles (which alone is
surprising enough), in fact, the referee seems to accept it as
proven. However the referee takes umbrage with the idea that such a
cusp trap can accelerate charged particles, and feels that a numerical
calculation should be required (rather than the analytic calculation I
used) to prove my assertion.
Whereas I can sympathize with the desire for ``computer'' proof, I
think it is neither necessary nor sufficient in this case.
(a) Not necessary, because the Earth already has a well-understood
trapping region in which acceleration is occurring continuously, the
radiation belts. Knowledge gained from the study of the 3 dipolar
adiabatic invariants is directly applicable to the cusp, once we have
established that there are 3 cusp invariants of the motion. Models of
the radiation belts that ``proved'' the occurrence of acceleration
did so without the benefit of computer simulations, using
only analytic representations of the fluxes. Once we have established
that the cusp has invariants, we can write down the acceleration
expected nearly analytically.
(b) Not sufficient, since the observation of the global cusp electric
field is in a very rudimentary state (in part due to the semi-random
location of the cusp), and would be simulated, more than likely,
inaccurately. That is, by giving the modeller free reign over the
electric field boundary condition, any acceleration desired could be
obtained on any timescale, without necessarily revealing the true
nature of the mechanism. As an experimentalist, I grow increasingly
skeptical of computer modelling as a means to predict any
non-empirical phenomena.
Now that I have vented my distrust of computer modelling, let me address
specifically the objections/requests made under this point.
3.1 Insufficient evidence for cusp acceleration?
On the contrary, by showing that the phase space density surrounding this
location show less particles without than within, one has direct empirical
evidence for in situ acceleration.
3.2 Insufficient evidence for cusp particles supplying ORBE?
The fact that no alternative theory exists suggests that any theory is
acceptable as long as it does not violate physical laws. Indeed, some
have seriously suggested Jupiter as the source of MeV electrons, a
theory which, unfortunately, did not hold up to scientific
scrutiny. Which is to say, the criteria for a valid scientific theory
is not certainty, but falsifiability; the ability to be shown false
(see Karl Popper's writings). Certainly this theory can be proven
false very easily: if the phase space density within the cusp is lower
than that in the radiation belts, then ``simple'' diffusion theories
would call it a sink rather than a source. We have shown that the
count rates are higher in the cusp, in our revised manuscript we show
that the phase space densities are higher as well. If another
falsifiable characteristic were identified, we would be glad to
subject our population to the test, but to claim insufficient evidence
is to imply a falsifiability criterion, which we assume are implied
in the following statements, and to which we reply below.
3.3 Test particles were traced in a T96 model without electric field?
The implication is that if we had included an electric field, we would
not have found trapped particles. This falsifiability criterion can be
easily dismissed by drawing on what we know of cusp electric fields.
The POLAR/TIDE experiment can measure the cold plasma flows within the
cusp. They found, during similar if not exactly the same cusp
crossings, a flow primarily in the field aligned direction, i.e.,
outflows, and only a slight breeze in the transverse direction. My
recollection of their findings, reported at the FALL AGU, were that 40
km/s flows were typical, which if we assume a 25 nT field, correspond
to about 1 mV/m.
If we assume that this is a global cusp DC field, then from the T96
model that I plot, we have about 30 kV across the cusp. Using a
Hamiltonian approach to the description of drift orbits, we know that
E-total = K.E. + P.E. = mu Bm + q U, where mu is the first invariant,
Bm is the mirroring field strength ($<$100 nT), q is the charge and U
the electrostatic potential described above. Then an increase of 30 kV
in potential from one side of the cusp trap to the other will decrease
the kinetic energy by 30keV. Since mu is conserved, this means that
Bm will have to decrease. Now this decrease cannot occur by changing
the pitchangle, since this would change mu as well, thus this only be
accomplished by having the drift orbit move toward lower $|B|$, e.g., by
moving cuspward. Since nearly all the energy in these trapped electrons
is in the perpendicular component, we can roughly say that a 100 keV
electron must move such that a 30\% decline in $|B|$\ is effected. If
we look at $|B|$\ in the cusp, we see that this can be accomplished quite
easily, since the cusp goes from 10-100nT over this range of
distances and has plenty of dynamic range to account for electric
field distortion of the drift orbits. Note that we have now traced
trapped orbits up to 800 keV, which have a proportionately smaller
distortion in their drift orbits due to a 30 kV potential.
3.4 The authors claim a large electric field is necessary?
The implication is that the DC electric field of 1 mV/m discussed
above is not sufficient to accomplish the acceleration of MeV
electrons. We do not claim that these electrons are accelerated by
purely adiabatic motion of the sort described in the previous
paragraph. Rather we argue for the necessity of resonant scattering
and/or AC electric fields, which can transfer energy to the electrons
by coupling to the 3 characteristic frequencies of the electrons, the
gyration, bounce and drift frequencies. Power at these frequencies is
available, and indeed, the EFI experiment on Polar shows broadband
electric field fluctuations with RMS exceeding 10 mV/m.
3.5 No quantitative evidence of sufficient energization is presented?
As we argue in point 3.2, lack of evidence implies a falsifiability
criterion. It is unclear what that criterion is. Do I need to demonstrate
a timescale, or an energy change or something else? Does ``sufficient''
mean that I need to illustrate how a keV electron gains an MeV? If so,
we have shown how the limiting case of ``Bohm diffusion'' illustrates
exactly this point. If I were to replace Bohm with ``Chorus
waves'' would this change the essential argument? If the referee could
be more precise, I could answer the criticism more concretely.
3.6 Perform test particle simulation with model electric field?
We are intending to do exactly that. I should point out that the test
particle tracing has been computing continuously for 6 months since
fall AGU and is now running 800 keV electrons. It takes 1 day of real
time computing to execute 300 seconds of cusp trapping. These
simulations are not quickly made. I have no doubt as to the outcome of
the simulations, but if this run would convince the referee, I would
be eager to run it. That is, I have limited resources of time and
computing, so that if the referee made this the \#1 priority, I will
run it immediately. The energy, however, will demonstrate purely
adiabatic effects unless I include stochastic electric fields. This
then runs into the whole question of boundary conditions. If the
referee will not fault me for my boundary conditions, I would again be
happy to include stochastic electric fields in the simulation. The
results will be entirely predictable and inconclusive, however, as I
have argued at the very beginning.
4) Arguments are largely conjecture?
It is difficult to separate the denotation from the connotation in
this paragraph. All hypotheses are conjecture. Does this invalidate
them? No, they wouldn't be hypotheses if they weren't. What would
invalidate a conjecture is a testable criterion (falsifiability) that
contradicts the predictions of a conjecture. We conjecture that Jan
10 was a fortuitous day for a magnetic cloud to accelerate electrons
because it was close to winter solstice, and the cusp was tipped
toward the sun. This seems to be both a valid hypothesis and easily
falsifiable.
When the referee writes ``Where is the proof?'' I am unclear as to
what aspect of the hypothesis he is objecting to. The proof that the
cusp is tipped toward the sun lies in solid geometry and knowledge of
the Earth's magnetic and spin axes. The proof that acceleration
occurred much faster than normal lies in the energy time plots from
GEO, HEO and Polar, showing a less than 12 hour acceleration
time. This is faster than normal if one uses the SEC MeV electron
prediction algorithm developed by Koons and Gorney 1991. It is also
faster than normal if one uses the many references (e.g., Baker et al,
1997) to a 24-48 hour time delay in the MeV electrons. The proof that
there was a causal relationship between the two is in the conjecture
presented, a simple chain of logical relations as follows: a) a cusp
that is tipped toward the sun has stronger magnetopause fields b)
stronger magnetopause fields lead to a larger volume of phase space
that is trapped in the cusp c) A large volume of phase space CAN lead
to a more efficient acceleration process d) Electrons that leak out of
the cusp may arrive in the radiation belts. e) A more efficient
acceleration process CAN lead to a faster rise in the ORBE.
All of these statements are falsifiable. All can be disproved. This then
is the ``proof''.
If in our paper we have not clearly outlined the logical steps above,
then the fault lies with us for having hidden the details of our
conjecture. If, however, the referee is demanding incontrovertible
proof of the validity of our theory, then we must object that this is
a goal unobtainable in the natural sciences.
5 [1]) SEU?
The inaccuracy is ours. If an SEU is defined to be a heavy ion disruption,
as it is typically used in the literature, we should not have applied it
to our MeV electrons. We were using it in the sense of a CMOS memory bitflip
that can occur because modern chips have shrunk the size of IC chips so much
that parts are becoming ``soft'' even to MeV electrons.
6 [2]) Selesnick phase space density profiles decline with increasing L?
I agree with the referee that if the phase space density declines with
increasing L, any normal diffusion model would predict a source at low-L.
Is this what Selesnick and Blake plot in figure 1 (GRL, vol 24, page 1348)?
Note carefully that the first column is differential flux, j, whereas the
second column is phase space density f, plotted at constant magnetic moment.
Orbit 283, (top panel) indeed shows a source at L=4. Orbit 439, however,
shows a source at L$>$7. Orbit 273, shows an annoying double source, one
at L=5 and one L>7.
How do we interpret these results? The abstract is clear: ``Radial
profiles of electron intensity and phase space density...show that
particle injections followed by fast radial diffusion lead to the main
part of the outer radiation belt near L=4 to 5 during active periods.
In quiet times the belt is supplied by inward diffusion from an
external source. Various combinations of these two basic
configurations are also seen.''
To rephrase, when the slope of f indicate a L$<$7 source, this can be
interpreted as the slow decaying remnant of a previous
injection. Since the loss or transport rate is faster at L=7 than at
L=6, one would expect the decay after an injection to produce a
negative slope with increasing L. Note that Selesnick carefully does
not say where the radial location of the ``injection'' mechanism is
located. It may also be located at large L-shells, and during the
first few minutes/hours of an event, send a wave of higher density
ORBE radially inward. When this wave is observed after some time, loss
processes would disguise its origin. Which is to say, steady state
diffusion models cannot describe a non-steady state injection mechanism
very well.
Regardless of the injection mechanism, Selesnick is in complete
agreement with us concerning the location of the quiet-time source,
which is exactly the time period for which the diffusion model is best
suited. Contrary to referee A's rebuttal, we find strong confirmation
in Selesnick's work for a large L-shell source and additional support
can be found in XinLin Li's work published earlier in GRL (vol 24,
page 924). Thus I strongly refute the referee's critique, and
emphatically reassert that no acceleration mechanism has been found
for MeV electrons within the dipolar trapping region itself.
7 [3]) Definition of 'major killer electron' event?
At first I did not understand the difficulty in our nomenclature, but
after rereading the referee's comment, I perhaps see the confusion.
If we insist on ``killer'' as a descriptive term for these electrons,
does this mean that by definition a ``major event'' should cause
spacecraft disruption? No, because the spacecraft could be radiation
hardened. Nor should the characteristics of the population we are
describing depend on the observational technique. ``Killer'' then only
refers to the energy of the electrons, not to their effectiveness in
any given storm. Then what is the meaning of a ``major killer electron
event''? Only that it be the highest recorded flux in comparison to
some baseline. I chose April 15-20 because it had the highest recorded
MeV flux for all of 1996.
8 [4]) Flux units?
The units have been added to the figures, we apologize for the omission.
Referee B:
1) figures?
We apologize for the print size in Fig 1. We can certainly rectify this
with judicious use of a graphics editing package. My version of the plot
shows that 01:00UT has an L-shell of 22.7, not 35, so perhaps the print
WAS too small. We will specify the fluxes in cts/s-sr-cm2 which can be
converted to phase space density if one knew the average energy response
of the channel, which turns out to be a ill-defined problem in
regions where the spectral index changes rapidly, hence our tendency to
only give counts.
We will expend some effort on figure 2, perhaps showing it in several perspectives to validate the ``lily orbits'' description.
I have been converted by Bob McPherron that all data should be shown
in solid lines, and theory in dashed or dotted, so this figure will be
changed to reflect the canonical plotting scheme. I will also try
using color to bring out the differences. The vertical scale is
relative, since the two data sets and one theory all use different
units.
2) Abstract?
We will change the abstract to better emphasize the scientific
content of this paper.
3) 29 year duration is duly noted
4) Li reference is updated
5) phase space density is computed and demonstrated in the plot.
6) The trapping in the T96 cusp existed for at least 300 seconds, or for
more than 4 complete drift orbits around the cusp. As we state, this
is without an electric field, though it is hard to imagine how an
800 keV electron would be detrapped by a 1 mV/m electric field, (as
we argue in our reply to Referee A). That is, these electrons appear
to be rather permanent residents of the cusp barring topological
changes to the magnetic field geometry.
7) We have elaborated on stochastic acceleration. We hope it has
answered more questions than it raises.
Sincerely,
Robert Sheldon