Mission to Understand Electron Pitch Angle Diffusion and Characterize Precipitation Bands and Spikes. J. F. Fennell 1 and P. T.

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1 Mission to Understand Electron Pitch Angle Diffusion and Characterize Precipitation Bands and Spikes J. F. Fennell 1 and P. T. O Brien 2 1 The Aerospace Corporation, MS:M2-260, P.O.Box 92957, Los Angeles, CA The Aerospace Corporation, MS:CH3-330, Conference Center Dr., Chantilly, VA Pitch angle diffusion into the atmospheric loss cone is a major cause of electron losses within the inner magnetosphere. When measuring the electron pitch angle distributions at the magnetic equator, it is difficult to assess whether pitch angle transport is uniform throughout the distribution or whether it is stronger over one part of the distribution relative to another. At moderately high L values, for example in the regions mapping near and above geosynchronous orbit, bands (see Fig. 1) of electron precipitation are commonly observed by low altitude polar orbiting spacecraft [Blake et al., 1996; Fritz, 1968; Vampola, 1971; Vampola, 1977]. Sometimes the magnetic field at these L values can become strongly stretched, especially during magnetically active and intense ring current periods, and the field s radius of curvature becomes small enough to cause loss of electron adiabaticity. This has also been observed, [Imhof et al., 1977, 1978, 1979, 1991]. Another kind of precipitation has been observed, called micro bursts [Blake et al., 1996, Lorentzen, et al., 2001a, 2001b; O Brien et al., 2003] that can extend over wide range of L values in the outer radiation zone. It is clear that not all the precipitation bands and bursts are generated by the same process. But, it is thought that the majority of the precipitation bands and microbursts observed at low altitudes are caused by wave-particle interactions. Figure 1. Example of multiple precipitation bands observed by SAMPEX during quiet conditions. The various panels indicate that the bands were observed from 25 kev to several MeV. Historical low altitude observations indicate that while the electrons in the precipitation bands are isotropic at low altitudes, the corresponding electron fluxes at the magnetic equator are relatively

2 unchanged (see below and Vampola, 1977). Koons et al [1972] reported a single case of pitch-angle isotropy over both loss cones at 4500 km, L 5.6, suggesting that the observing satellite was in the actual scattering region. Coincident with the scattering were strong electrostatic waves from 400 Hz to 7.4 khz, possibly proton cyclotron frequency waves Doppler shifted by the satellite motion. Koons et al. [1972] estimated that the electrostatic wave power was sufficient to drive the electrons to strong pitch angle diffusion. This implied that the precipitation bands were low altitude phenomena wherein the strong waves essentially enlarged the loss cone for the electrons while leaving the equatorial distribution unaltered. Figure 2. [a] Superposed epoch analysis of high altitude HEO3 fluxes taken during observations of precipitation bands at SAMPEX altitudes. [b] Spatial occurrence of precipitation bands in L versus MLT for four different levels of magnetic activity defined using D ST ranges.

3 Figure 2a shows the spatial distribution of precipitation bands, observed by SAMPEX, in L versus MLT at four different levels of magnetic activity specified by D ST. During the quite and low activity periods the SAMPEX precipitation bands are localized at the higher L values ( 5) on the night side, consistent with the early observations (Vampola, 1977). During moderate and high activity levels the occurrence of precipitation bands expands to earlier and later local times and to lower L values. Figure 2b shows a superposed epoch analysis of HEO3 observations, taken during the same time frame that SAMPEX precipitation bands occurred, which shows that the high altitude fluxes were relatively unperturbed on average. These more recent observations are consistent with Vampola s [1977] earlier conclusion. The question is, can we really explain all these observations where evidence of strong pitch angle scattering is often observed, out to the trapping boundary, at low altitudes while little response occurs closer to the equator? How do we provide a quantitative explanation of the scattering and loss? Is the whole electron angular distribution isotropized or only a portion of it consisting of the electrons with pitch angles near the loss cone? How can we tell? The questions beg for an observational mission to answer them once and for all time. We outline the requirements for a possible satellite mission below. Satellite Mission Requirements What are the requirements for a satellite mission to examine the electron precipitation causes and their characterization? Taking the historical and most current observations as our guide, it is clear that we need a mission to study the electron loss cone and near loss cone angular distributions. But we need to do this close up and not from the magnetic equator where it is difficult to resolve the loss cone with modest instrumentation and obtain statistically significant electron flux samples. Selesnick et al. [2003] showed the great potential of measurements of both the drift and bounce loss cones for understanding radiation belt electron loss, but, due to the orbit and instrument limitations of SAMPEX, they were required to make several crucial assumptions that could not be verified. Additionally, we would prefer to not have a sun synchronous orbit so that local time sampling can be achieved at all L values to characterize the MLT dependence of the electron scattering for comparison with possible mechanisms (e.g., EMIC versus CHORUS scattering) and previous observations such as those in Fig. 2a. A simple satellite configuration would be a spinning satellite with its spin axis perpendicular to the magnetic meridian plane to provide the best viewing of the pitch angle distributions for particle sensors mounted perpendicular to the spin axis. The vehicle could have solar arrays on all sides or be much like the S3-2, S3-3 and POLAR satellites that had arrays on the sides and one end. The satellite could be reoriented using magnetic torquing coils to maintain the sun on the arrays and the spin axis perpendicular to the meridian plane of the field. Examples of such vehicles are the USAF S3-2 and S3-3 satellites, which used torque coils, and NASA s POLAR satellite, which used a gas system, for reorienting their spin axes. The spacecraft spin rate should be optimized to give relatively rapid sampling of the complete 2-D angular distributions. Roughly a 5 second spin period would be a good compromise. To cover all the L values involved a satellite needs to have a high inclination orbit that passes in latitude from above the poleward edge of the radiation belt, through the outer radiation zone and down through the slot region (i.e., from > L > 2). At the same time, the orbital altitude of the spacecraft must be high enough to sample pitch angles that are sufficiently outside the bounce and drift loss cones so as to obtain a good reading of the pitch angle distribution shape away from the loss cone. Simultaneously it must be at low enough altitude that the loss cone is wide enough to be resolved by relatively simple particle instruments. As an example, for a mission with an altitude of ~8000 km the local pitch angle corresponding to the loss cone is ~ Such a loss cone would be mappable with simple particle instruments such as those to be flown on RBSP. The loss cone would represent 30-45% of the local pitch angle coverage. Thus there would be a sensitive measurement of the particle distributions approaching and into the loss cone. Such a measurement would also resolve the drift and bounce loss cones most of the time, enabling observation of fast (bounce timescales) and moderate (drift timescales) loss processes.

4 However, the satellite orbit need not be circular. A high inclination elliptical orbit with an apogee of 8000 km or slightly greater would also work, especially if the orbit precesses (as it most likely will) such that a range of altitudes are obtained at each L value traversed over the mission life. Such orbits can be achieved by piggybacking as a secondary payload on a launch with excess capability, for example using an ESPA ring. In addition to energetic electron sensors for measuring the electron pitch angle distributions over a 20 to 2000 kev energy range, the satellite should also carry a good plasma electron/ion sensor, that performs well in penetrating electron fluxes, energetic ion spectrometers to measure, as a minimum, the protons from 10 s to 1000 s of kev, a science quality magnetometer, a plasma wave experiment and a plasma density measurement. The wave experiment should provide at least a two-axis measurement of wave E and three-axis measurement of wave B over the full VLF range to cover from ion cyclotron frequencies to beyond the electron gyrofrequency. The magnetometer needs to sample at a high enough rates to provide overlap with the wave experiment on the low frequency end, cover the LF, ULF, and ELF frequencies and provide field DC vectors at rates sufficient for the particle and plasma sensors to obtain good pitch angle resolution. Such a compliment of instruments could provide all the measurements necessary to investigate the electron pitch angle diffusion near the loss cone and determine whether it is a local or remote (high on the field line) process. The instruments will also provide measurements of the background plasma conditions necessary to the theory and modeling needed to interpret the electron observations. The wave measurements, of course, are necessary to confirm the existence or lack thereof of local particle scattering near and just above the loss cone. The proton measurements identify ion precipitation related to EMIC generation and ring current injection and penetration to low L values that also play a role in electron losses, such as the enhanced field curvature related scattering noted above. As secondary science, such a mission would be able to investigate the question of how the inner zone proton belt responds to losses from atmospheric inflation during solar activity and then refills, and how solar particles are entrapped to become an extension of the inner zone to higher altitudes and then dumped during storm activity. In particular, the mechanisms of trapped proton diffusion to refill particles lost during an atmospheric inflation event are largely unknown. Selesnick et al. [2007] omitted pitch-angle scattering entirely from their long-term theoretical simulation of the inner belt, while Looper et al., [2005] showed that some kind of diffusion refills the low altitude extent of the belt (after a storm) over a period of months. Unknown diffusion mechanisms observable from low altitude likely have implications for redistribution of trapped protons at higher altitude, which, in turn, controls the lifetime of protons throughout the inner belt. No formal costing and mass estimates can be made without expending funds and more time than was available for generating this white paper. However, one can make a reasonable assessment that the mission proposed falls in to the small mission (SMEX like) category for both cost and mass. The cost can be constrained by using copies of sensors that were developed for other missions such as RBSP and THEMIS. The mass is constrained by limiting the number of sensors to a basic set just sufficient to make the measurements required to do the science.

5 References: Blake, J.B., M.D. Looper, D.N. Baker, R. Nakamura, B. Klecker, and D. Hovestadt, New high temporal and spatial resolution measurements by SAMPEX of the precipitation of relativistic electrons, Adv. Space Res. 18(8), 171, Fritz, T. A., High-latitude outer-zone boundary region for >40-keV electrons during geomagnetically quiet periods, J. Geophys. Res., 73, , Imhof, W.L., J.B. Reagan, and E.E. Gaines, Fine scale spatial structure in the pitch angle distributions of energetic particles near the midnight trapping boundary, J. Geophys. Res., 82, 5215, Imhof, W.L., J.B. Reagan, and E.E. Gaines, High-resolution study of the spatial structure in the pitch angle distributions of energetic particles near the midnight trapping boundary, J. Geomagn. Geoelec., 30, 467, Imhof, W.L., J.B. Reagan, and E.E. Gaines, Studies of the sharply defined L dependent energy threshold for isotropy at the midnight trapping boundary, J. Geophys. Res., 84, , Imhof, W.L., et al., The precipitation of relativistic electrons near the trapping boundary, J. Geophys. Res., 96, , Koons, H.C., A.L. Vampola, and D.A. McPherson, Strong pitch angle scattering of energetic electrons in the presence of electrostatic waves above the ionospheric trough region, J. Geophys. Res., 77, 1771, Looper, M. D., J. B. Blake, and R. A. Mewaldt (2005), Response of the inner radiation belt to the violent Sun-Earth connection events of October November 2003, Geophys. Res. Lett., 32, L03S06, doi: /2004gl Lorentzen, K.R., J.B. Blake, U.S. Inan, and J. Bortnik, Observations of relativistic electron microbursts in association with VLF chorus, J. Geophys. Res., 106, , 2001a. Lorentzen, K. R., M. D. Looper, and J. B. Blake, Relativistic electron microbursts during the GEM storms, Geophys. Res. Lett., , 2001b. O Brien, T. P., K. R. Lorentzen, I. R. Mann, N. P. Meredith, J. B. Blake, J. F. Fennell, M. D. Looper, D. K. Milling, and R. R. Anderson, Energization of relativistic electrons in the presence of ULF power and MeV microbursts: Evidence for dual ULF and VLF acceleration, J. Geophys. Res., 108(A8), 1329, doi: /2002ja009784, O Brien T. P., M. D. Looper, and J. B. Blake (2004), Quantification of relativistic electron microbursts losses during the GEM storms, Geophys. Res. Lett., 31, doi: / 2003GL Selesnick, R. S., J. B. Blake, and R. A. Mewaldt, Atmospheric losses of radiation belt electrons, J. Geophys. Res., 108(A12), 1468, doi: /2003ja010160, Selesnick, R. S., M. D. Looper, and R. A. Mewaldt (2007), A theoretical model of the inner proton radiation belt, Space Weather, 5, S04003, doi: /2006sw Vampola, A.L., Electron pitch-angle scattering in the outer zone during magnetically disturbed times, J. Geophys. Res.,76, , Vampola, A.L., The effect of strong pitch angle scattering on the location of the outer-zone electron boundary as observed by low-altitude satellites, JGR, 82, 2289, 1977.