Observation of 40keV Field-Aligned SubAuroral Ion Beams

Robert B. Sheldon1, Harlan E. Spence1 and Joseph F. Fennell2

1Boston University Center for Space Physics
725 Commonwealth Av, Boston, MA 02215
2The Aerospace Corporation
El Segundo, CA
18 June 1997

Abstract

Abstract

Recent observations by CEPPAD/IPS from the new perspective of the POLAR orbit reveal a close association of field-aligned ionospheric beams with convecting plasmasheet "nose" ions. April 15, 1996 was marked by a large southward Bz accompanying a fast solar wind shock. This event triggered a strong cross-tail convection electric field that pushed plasmasheet ions deep into the magnetosphere. When the POLAR spacecraft passed through the inner magnetosphere, it observed both an unusually energetic "nose" plasmasheet ion injection at ~90 keV, as well as a peculiar field-aligned beam at ~40 keV. These beams appear to be dominated by oxygen ions, which would place their origin in the ionosphere. Both populations existed from L=7--3, for a duration of at least 2 hours. We speculate that the nose ions create a parallel electric field that is responsible for the extended ionospheric beam signature.

Data Analysis

The POLAR spacecraft is in a polar, 9 x 2 Re orbit that in April 1996, was in the noon-midnight meridian plane with perigee over the south pole, making two diagonal passes through the midnight and noon radiation belts. The Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD) (Blake et al. [1995]) IPS experiment, consists of a multiple solid state detectors that detect ions between 15--1500 keV in 9 elevation angles and 16 or more azimuthal bins. A typical pass on April 14, 1996, (see fig 1 panel a), shows an energetic particle population whose average energy is proportional to the |B|.

This is the normal distribution for ions diffusing in L-shell from a source region in the plasmasheet (Sheldon [1993b]), showing the adiabatic energization expected from Liouville's theorem. On April 15, however, IPS detected two nearly monoenergetic bands superposed on the night side ring current (panel b) at ~90 keV, and at ~40 keV. Unlike the ring current, these two bands dropped to lower energies with lower L-shell, displaying an apparent non-adiabatic behavior. The bands became less distinct on the dayside and more sporadic, but apparently exist throughout the ring current. To better understand these ions, we examine only the lowest 8 energy channels and display the roll modulation as the spacecraft rotates parallel and perpendicular to the magnetic field (fig 2).

POLAR/CEPPAD/IPS data on (a) April 14, and (b) April 15, and (c) April 16, 1996, displaying energy spectra summed over pitch angles from 20--1500 keV.

Nose Ions

We observed that the upper, ~90 keV band is strongly peaked around 90o and the lower band is peaked near 0o. Now a monoenergetic trapped population is possible when a strong cross-tail electric field drives ions against the grad B drift deep into the magnetosphere (Smith [1974]). Such a "nose" event must be nearly 90o trapped particles because of the large increase in |B| while convecting from the plasmasheet, which is the characteristic of the upper energy band in our data. Since nose events are highly correlated with storms, and storms are defined by Dst, we turn to the preliminary Dst provided by the Kyoto University web site.

After subtracting the ionospheric Sq contribution using the "quietest day of the month" (April 7/8) method, we find a moderate storm of at least -63nT on the first hour of April 15. More significantly, we find a very large asymmetric (ASY) component on that day. Examination of the one-minute symmetric H components show that a sudden increase preceded the storm decrease, thereby cancelling most of the storm onset in Dst. This missing current shows up clearly in the ASY component, leading us to believe that a stronger than normal electric field had distorted the ring current, shifting the index from symmetric to asymmetric, and disguising the magnitude of this storm in Dst. Additional support for the nose event identification with a storm injection came from the extensive GGS database.

POLAR/CEPPAD/IPS data on April 15, 1996, displaying roll modulation of the counts in the 90o head in the energy bands from 24--138 keV. Dotted white and blue lines are 90 and 60o pitchangles.

Examination of WIND data (fig 3.)Ogilvie et al., [1995]) for April 14 showed that there were several Bz<0 periods lasting for 1 hour or less. Corresponding CU and CL derived from the CANOPUS array (Rostoker et al. [1995]) show that these periods led to substorms with riometer absorption signatures at auroral latitudes. However the storm trigger appears to be the strong southward turning of Bz < -10 occurring at 2000 UT, accompanied by a jump in the solar wind speed from 450 km/s to 600 km/s, which produced an even larger Bz in the compressed magnetic field of the magnetosheath. This period of strong southward Bz lasted more than 3 hours, effectively saturating the ability of the tail to shield out the polar cap potential. The IZMIRAN model (Papitashvili [1994]) predicted in excess of 150 kV across the polar cap for these solar wind conditions.

April 14-15, 1996 ISTP data. CANOPUS (a) CU (b) CL and riometers at (c) L=12, (d) L=8, (e) L=6.6, and (f) L=4.4. (g) WIND/MFI Bz and (h) WIND/SWE solar wind speed.

The CANOPUS array detected a magnetic bay, a nearly equal response of AU and AL, suggesting that the current systems had moved equatorward, overhead of the magnetometers. Indeed, the Halley Bay magnetometer at L~4 (Dudeney et al. [1995]) showed a large H deflection with almost no Z deflection, indicative of strong overhead currents. The ionosphere responded strongly at this time (private communication, J. Aarons, 1996) as ionospheric scintillations were enhanced. While CANOPUS riometers at L > 6 recorded very little activity, the riometer at L=4.4 as well as the Halley Bay riometer at L~4, (private communication A. Rodger, 1996) showed an extremely intense and narrow absorption feature at this time, indicating that precipitation had penetrated to low latitudes, deep in the magnetosphere and down to E-layer ionospheric depths.

From these observations, as well as complementary ground observations, we surmise that after several intense substorms had pumped up the plasmasheet, a strong convection field injected the plasma to at least L=4.4, which POLAR/CEPPAD observed as a 90 keV band. The large asymmetric component of Dst is evidence that this current was observed by the low latitude stations.

Field-Aligned Beam

The band of 40 keV field-aligned ions, however, are harder to explain. They have the wrong pitchangles to have convected from the plasmasheet, because the adiabatic decompression involved in backtracing them to their origin would place them in the plasmasheet loss cone. But ions of this energy would not have had access to the plasmasheet simultaneously with higher energy ions, since the magnetosphere is a "notch filter" for only one energy. This implies that these ions are trapped on closed drift orbits. But if they undergo the same processes as the adiabatically energized ring current, which can be seen simultaneously with the banded distribution, they would not be monoenergetic, nor would they track the energy of the nose ions so precisely. That is, if they had resided for any length of time in the magnetosphere, the same convection that brought in the plasmasheet ions would disperse these ions as well.

If we examine the loss cones in high time resolution, (though our spatial resolution remains 12 x 22 degrees) it is clear that one loss cone is substantially more filled up than the other, which is inconsistent with any mechanism that relied on convecting ions. Thus we conclude that these field-aligned ions are in situ accelerated during the time of the measurement.

The composition experiment, POLAR/CAMMICE, was turned off during this period, so we cannot determine the E>30keV O+ content for April 15, however when it was switched back on, around April 19, it found an anomalously large amount of O+ in the ring current (RC) in two energy bands centered at 40 and 100 keV. Because of the short lifetime of O+ against charge exchange, such a measurement is consistent with a storm injection occurring only a few days previously, since between storms, CAMMICE detects no O+ enhancements. A similar storm on March 21 showed that the IPS field-aligned beams are simultaneous with the double peaked O+ spectrum observed by CAMMICE. POLAR/TIMAS (Shelley et al. [1995]) (private communication W.K. Peterson, 1996) did detect E<20keV O+ beams for this day.

Therefore we conclude that on April 15 we are observing O+ ions accelerated to ~30--40 keV in field-aligned beams, presumeably by strong parallel electric fields in the ionosphere. Since these ions track ~50 keV below the nose ions, it appears there must be a causal connection, which becomes more apparent when we fit the spectral shape of these populations.

Peak Fits

The three spectral peaks all have differing shape so that we have used three separate functional forms to fit them. Adiabatic energization and diffusion explain the highest energy ring current peak, which we are able to fit very well with a Gaussian in log(Energy) space. The middle "nose" ion peak is represented in our instrument by a response only one or two channels, which does not constrain the functional form very much. We have chosen to fit this peak with a Gaussian in linear space simultaneously with the lowest energy "beam" peak. The beam reflects a highly asymmetric shape with a long tail toward lowest energies but a sharp cutoff at high energies. After much experimentation, we found that a Chapman layer function, described as y=exp(1+x-exp(x)), gave the best fit with the fewest number of free parameters. We have thus fit 16 energy channels with 9 parameters, carrying out this fit for 96s averages throughout the nightside pass, fig 4.

Successive 96s spectra (symbols) and three peak fit (line) for April 15 data set.

From the fits shown above, we plot the center energy and the width of the peak in figure 5. to demonstrate how the three populations, beam, nose and RC, evolve during this orbit. The RC is adiabatically energized so that as the model equatorial magnetic field increases the energy increases, as noted by the squares and dashed line. A latitude effect is apparent in the RC due to the more rapid loss of low energy ions at higher latitudes. This energization is not seen by the nose ions because they are not an equilibrium trapped population, rather we are seeing the subset of plasmasheet ions that have convective access to this location, so that stronger B-field ions also began with smaller magnetic moment (Sheldon [1994b]). The 90 keV energy of the nose ions is consistent with a large polar cap potential. Most importantly, we find that the ion beam energy, though broader because of the asymmetric shape, tracks the beam energy very precisely when we plot it as a ratio. That is, rather than a constant energy difference, the beam is a constant fraction of the nose ion energy, as shown by the X's in the above figure. This is a most important clue, since the nose energy is completely determined by details of the magnetic topology and convection electric field, whereas the beam is a local, in situ phenomenon.

Peak fits to 96s averaged spectra of the 90o head. Ring current ([]), nose ion (*), beam ion (triangle) energies, and the ratio of nose/beam X10 (X) are plotted. Error bars are FWHM from peak fits, since fitting errors are negligible. Overplotted dashed line is the model equatorial magnetic field intensity logarithmically scaled from 20--1000 nT; and the model magnetic latitude (dotted line) linearly scaled from -40 to 40 degrees, with the equator marked with a vertical dotted line.

Discussion and Conclusions

Since these ion beams track the nose ions so closely, we assume that the acceleration process must involve an in situ parallel electric field that is somehow related to the nose ions. Alfven's pioneering work on parallel electric fields comes to mind (Alfven [1963]) (though we prefer the treatment of Whipple [1977]) where he argues that one can produce a field-aligned electric field if the electron and ion phase space densities are not identical. Using this event as a guide, a 90 keV monoenergetic nose ion population of ions with equatorial pitch angles less than 90o (Sheldon [1994b]) was superposed on a cold plasmasphere electron population of very different pitch angles. According to (Whipple [1977]), this would produce a parallel electric field of several kT_e pointing toward the equator. That is, since a PA < 90o ion spends most of its time away from the equator, the electrons at the equator will experience a force pulling them toward higher latitudes so as to shield the ion charge, and ions will be repelled toward the equator. This then, is the correct sign of the electric field to account for the ion beams observed by POLAR near the equator. Thus it appears that a parallel electric field is a natural consequence of storm injections, and may provide an important link in the storm progression. The presence of oxygen in the beam suggests that this parallel field penetrates down to nearly ionospheric depths and may play an important part in the development of Dst during storms. A more detailed theory will be described in a later paper.

Acknowledgements

This study was supported by NASA contract NAS5-30368 and NSF grant ATM-9458424. The authors gratefully acknowledge K. Ogilvie, E. Shelley, J. Dudeney, G. Rostoker, B. Blake and T. Fritz for the use of their data.

References

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