University of New Hampshire, Durham.
Abstract
Velocity dispersed ionospheric O+ and He+ ions originating at the magnetopause have been observed by the FAST satellite in the polar cusp. These ions, together with H+ ions, were observed during the September 25, 1998 magnetic storm when the magnetosphere was filled with ionospheric plasma. This is the first published observation of dispersive O+ and He+ bursts in the cusp. Velocity dispersed H+ and He++, with a signature of decreasing energy with increasing latitude, are normally observed in the cusp. These ions originate in the magnetosheath and, after field line reconnection, are accelerated by the JxB force at the magnetopause appearing as dispersive bursts in the low altitude cusp. Northward convection of the plasma leads to the latitude-velocity dispersion signature. Magnetospheric plasma can also be accelerated at the magnetopause, however the magnetosheath plasma density is much larger and magnetospheric composition is normally dominated by H+ so that distinguishing magnetospheric and magnetosheath plasma becomes difficult. During the September 25 magnetic storm, ionospheric outflow of plasma was large enough to fill the magnetosphere and produce this atypical cusp signature.
Introduction
Magnetic storms represent one of the main perturbations to the Earth's magnetosphere and, as such, are important to understanding the dynamics of the near Earth environment. Measurements by Hamilton et al. [1988] and later by Daglis [1997] have demonstrated that for large magnetic storms, the ring current is dominated by ionospheric plasma, with a large contribution from O+ during the storm's maximum phase. Since the charge exchange time for O+ at >40 keV energies is faster than for H+, the presence of large amounts of O+ explains the observation initially made by Akasofu et al. [1963], that Dst for great magnetic storms shows a two stage decay [Hamilton et al., 1988]. How O+ is energized and becomes a major constituent of the ring current is still under debate, however it seems likely that the ring current O+ was part of the plasmasheet at an earlier time. Statistical studies show that during active times the plasmasheet composition changes and that O+ becomes a significant component [Lennartsson and Sharp, 1982; Lennartsson and Shelley, 1986; Daglis et al., 1994].
The enhanced O+ in the magnetosphere should have other effects on the dynamics of the near Earth environment. One consequence is that dayside reconnection will be affected by a the presence of a heavy constituent. Under normal conditions, the dayside magnetospheric composition is primarily H+ and its density is much smaller than the magnetosheath. Spacecraft crossing the magnetopause often see jets of ions [Paschmann et al., 1979] that have been accelerated by the JxB force at the magnetopause. (For more on magnetopause reconnection see Cowley, 1995 or Fuselier, 1995, and references therein.) Because the magnetosheath density is higher, the observed jets are dominated by magnetosheath plasma. The most field aligned and Earthward moving of these ions eventually travel down into the cusp producing a latitude-velocity dispersion signature at low altitudes. The dominant component of these ions is always H+, although He++ can be detected when a sensitive enough mass spectrometer is present on the spacecraft. O+ ions have been reported in the cusp [Peterson, 1985], but do not show a dispersive ion signature. In this brief report we present observations of velocity dispersed, earthward travelling O+, He+ and H+ in the cusp during the September 25, 1998 magnetic storm. This rare event was observed because the magnetosphere was filled with a high concentration of ionospheric plasma during the magnetic storm. This allowed the magnetospheric component which was accelerated at the magnetopause to be both observable and distinguishable from the magnetosheath component.
Observations
The magnetic storm on September 25, 1998 was initiated by a CME that reached the Earth ~15 minutes before the start of the day. The Dst initially dropped to -85 at 3 UT, reached a minimum of -207 at 10 UT, and recovered to -50 by the end of the day. The initially large By and later large Bz in the solar wind led to enhanced reconnection on the dayside as reported by Strangeway et al. [2000] and to enhanced magnetospheric convection as indicated by AE>500 for most of the first 2/3 of the day. Large outflows of O+ from the cusp were reported by Moore et al. [1999] and Strangeway et al. [2000]. These outflows were energetic enough to provide escape for O+ into the mantle, with magnetotail reconnection likely adding this plasma to the plasmasheet. Injection of ionospheric O+ and He+ from the nightside oval directly into the plasmasheet is also likely. An intense substorm initiated by the arrival of the CME brightened most of the nightside oval and shifted the polar cap boundary at midnight to ILAT~82o. AE rose to >1000 several times during the course of the storm indicating additional substorms occurred. It has long been known that ionospheric outflow increases with activity.
At about ~2:50 UT, during a nightside auroral crossing, the FAST satellite measured plasmasheet ions (<10 keV) that were dominated by O+ and contained significant amounts of He+. On the following orbit, O+ and He+ became important constituents of the cusp precipitation at low altitudes. The FAST satellite had a noon-midnight orbit during this period, producing a latitudinal cut through the polar cusp at a 12:55 MLT (4:28 UT). Velocity dispersed downgoing ions were observed as is usual for this type of cusp crossing, with lower energy ions appearing poleward. This particular pass was unusual because three separate peaks of velocity dispersed ions were observed separated by factors of four in energy. Mass spectrometer data clearly identify the higher and lower peaks as O+ and H+, with the intermediate He+ peak not resolved by the sensor due to slow sampling.
Figure 1 shows energy-time spectrograms of ions measured by the FAST satellite as it crossed the northern polar cusp at ~4:30 UT on September 25, 1998. The top panel shows the downgoing ions measured by ion plasma sensor. Three dispersive peaks can be seen between 4:27:55 and 4:28:45 UT and at energies of about 0.2-0.6 keV, 1-2.5 keV and 5-10 keV. A second set of dispersive bursts of ions between 4:29:05 and 4:29:35 indicates that pulsed reconnection [Lockwood et al, 1998] was taking place at the magnetopause. The second panel shows the upgoing ions which were primarily conic outflow of O+. The bottom panels show the count rates from the O+ and H+ channels of the TEAMS mass spectrometer which allow identification of the downgoing higher and lower energy dispersive peaks as O+ and H+. The intermediate energy dispersive burst in panel 1 was unresolved by the mass spectrometer due to slower sampling. This burst is consistent with He+ since factors of four in the energy per charge ratio between the dispersive peaks should be the mass ratios for ions with the same velocity and charge.
Figure 2a shows the energy-angle distribution of the ion energy flux during the dispersed ion event. Three peaks can be seen in the downgoing distribution, with pitch angle dispersion causing the peaks to fold to higher energies with larger pitch angle. Figure 2b shows slices through the downgoing energy flux distribution at several pitch angles which clearly shows the peaks. The three flux peaks represent phase space densities of about 6x10-23, 10-23 and 10-23 s3/cm6 for the H+, He+ and O+ peaks, respectively.
Summary
Measurement of velocity dispersed ions in the polar cusp is almost always dominated by H+. When field lines reconnect, the magnetosheath and magnetospheric plasmas mix and are accelerated by the JxB force at the magnetopause. The accelerated plasma is observed in the low altitude cusp as velocity dispersed ions, with decreasing energy with increasing latitude. The dominance of H+ in these events is due to the generally much higher density the magnetosheath plasma over magnetospheric plasma on respective sides of the magnetopause. When a second constituent is observed, it is normally He++ from the magnetosheath. In addition, the dominant dayside magnetospheric constituent is also normally H+ and can't be distinguished from the magnetosheath H+. However, during strong magnetic storms the magnetosphere is filled with ionospheric plasma. This plasma will be accelerated by JxB forces at the magnetopause and appear as a downgoing dispersive flux in the low altitude polar cusp. We report the first observation of downgoing velocity dispersed O+ and He+ ions in the cusp, in association with the magnetic storm on September 25, 1998. These ions are observed because the storm associated ionospheric outflows have filled the magnetosphere with ionospheric plasma. The presence of significant heavy ions at the magnetopause may have implications on reconnection rates and energy flow.
References
Akasofu, S.-I., S. Chapman, and D. Venkatesan, The main phase of great magnetic storms, J. Geophys. Res., 68, 3345, 1963.
Cowley, S. W. H., Theoretical perspectives of the magnetopause: A tutorial review, in Physics of the Magnetopause, Geophys. Monogr. Ser., vol. 90, edited by P. Song., B. U. O. Sonnerup, and M. F. Thomsen, AGU, Washington D. C., pp. 29-43, 1995.
Daglis, I. A., S. Livi, E. T. Sarris, and B. Wilken, Energy density of ionospheric and solar wind origin ions in the near-Earth magnetotail during substorms, J. Geophys. Res., 99, 5691, 1994.
Daglis, I. A., The role of magnetosphere-ionosphere coupling in magnetic storm dynamics, in Magnetic Storms, Geophys. Mono. 98, 107, 1997.
Fuselier, S., Kinetic aspects of reconnection at the magnetopause, in Physics of the Magnetopause, Geophys. Monogr. Ser., vol. 90, edited by P. Song., B. U. O. Sonnerup, and M. F. Thomsen, AGU, Washington D. C., pp. 181-187, 1995.
Hamilton, D. C., G. Gloeckler, F. M. Ipavich, W. Studemann, B. Wilken, and G. Kremser, Ring current development during the great geomagnetic storm of February 1986, J. Geophys. Res., 93, 14,343, 1988.
Lennartsson, W., and R. D. Sharp, A comparison of the 0.1-17 keV/e ion composition in the near equatorial magnetosphere between quiet and disturbed conditions, J. Geophys. Res., 87, 6109, 1982.
Lennartsson, W., and E. G. Shelley, Survey of 0.1- to 16-keV/e plasma sheet ion composition, J. Geophys. Res., 91, 3061, 1986.
Lockwood, M., C. J. Davis, T. G. Onsager, and J. D. Scudder, Modeling signatures of pulsed magnetopause reconnection in cusp ion dispersion signatures seen at middle altitudes, Geophys. Res. Lett., 25, 591, 1998.
Moore, T. E., W. K. Peterson, C. T. Russell, M. O. Chandler, M. R. Collier, H. L. Collin, P. D. Craven, R. Fitzenreiter, B. L. Giles, and C. J. Pollock, Ionospheric mass ejection in response to a CME, Geophys. Res. Lett., 26, 2339, 1999.
Paschmann, G., B. U. O. Sonnerup, I. Papamastorakis, N. Sckopke, G. Haerendel, S. J. Bame, J. R. Asbridge, J. T. Gosling, C. T. Russell, and R. C. Elphic, Plasma acceleration at the Earth's magnetopause: Evidence for magnetic reconnection, Nature, 282, 243-246, 1979.
Peterson, W. K., Ion injection and acceleration in the polar cusp, in The Polar Cusp, ed. J. A. Holtet and A. Egeland, pp. 67-84, 1985.
Strangeway, R. J., C. T. Russell, C. W. Carlson, J. P. McFadden, R. E. Ergun, M. Temerin, D. M. Klumpar, W. K. Peterson, T. E. Moore, Cusp field-aligned currents and ion outflows, J. Geophys. Res., submitted, 2000.
Figure 1. Ion energy-time spectrograms during the cusp crossing on orbit 8278 by the FAST satellite that shows latitude-velocity dispersion of H+, He+ and O+. The top panel shows three velocity dispersed bursts of downgoing ions at 4:28 UT, with the higher and lower energy bursts identified as O+ and H+ from panels 3 and 4. The second panel shows upgoing ions which are predominantly low energy O+ conics. Panels 3 and 4 show the unidirectional count rates for O+ and H+ ions as measured by the TEAMS mass spectrometer.
Figure 2. Figure 2a is a contour plot of the ion differential energy flux (proportional to count rate) during the velocity dispersed ion event in Figure 1. Three separate peaks at about 0.6 keV, 2.5 keV and 10 keV can be identified as H+, He+ and O+. Figure 2b shows energy flux spectra over downgoing pitch angles <45o which clearly show the peaks.
