Los Alamos Magnetospheric Plasma Analyzer Research Articles

Superposed epoch analysis of a whistler instability criterion at geosynchronous orbit during geomagnetic storms

Enhanced whistler mode waves produced by anisotropic hot plasma-sheet electrons outside the storm-time plasmapause have been suggested as one mechanism for accelerating relativistic outer-belt electrons in the aftermath of geomagnetic storms. Using measurements from the Los Alamos Magnetospheric Plasma Analyzers in geosynchronous orbit, we perform a superposed-epoch study of the storm-time behavior of the inferred plasma-sheet whistler growth parameter. Separate analyses are done for storms that result in strong relativistic electron enhancements and those that do not. The inferred whistler instability is strongest in the midnight-to-dawn sector, where freshly injected plasma-sheet electrons drift into and through the inner magnetosphere. During the main phase of both sets of storms, there is a marked drop in the whistler growth parameter, especially in the prime midnight-to-dawn sector. In the early recovery phase, this parameter is elevated and then returns to more typical values over the next few days. The elevation of the whistler growth parameter persists longer for the electron-enhanced storms than for those that do not produce such enhancements. These results suggest that whistler wave generation is greater during storms yielding enhanced levels of relativistic electrons.

A statistical comparison of hot‐ion properties at geosynchronous orbit during intense and moderate geomagnetic storms at solar maximum and minimum

Hot‐ion measurements at geosynchronous orbit from the Los Alamos Magnetospheric Plasma Analyzer (MPA) instrument during geomagnetic storms at solar maximum (July 1999–June 2002) and at solar minimum (July 1994–June 1997) are collected, categorized, and analyzed through the superposed epoch technique. To investigate this source of the storm‐time ring current, the local time (LT) and universal time (UT) dependence of the average variations of hot‐ion fluxes (at the energies of ∼30, ∼17, ∼8, and ∼1 keV), density, temperature, entropy, and temperature anisotropy are examined and compared among four storm categories, i.e., 44 intense storms and 120 moderate storms, defined by the pressure corrected Dst (Dst*), at the two solar extrema. All the hot‐ion parameters are highly disturbed around Dst*min; they show distinct peaks or minima and display obvious increase or decrease regions, whose locations do not change much with levels of geomagnetic activity and solar activity. It is also found that intense storms at solar minimum always have the highest (lowest) average peak value (minimum) in each hot‐ion parameter. Around Dst*min in each storm category, hot ions are clearly denser near dawn than those near dusk. On the nightside and in the afternoon sector, temperature and entropy during solar minimum storms are usually higher than those during solar maximum storms; there is actually no clear temperature and entropy enhancement during solar maximum storms. During each type of storm, hot ions are isotropic on the nightside but anisotropic (Tper/Tpar > 1) close to noon.

Observations of bow shock and magnetopause crossings from geosynchronous orbit on 31 March 2001

On 31 March 2001, significant enhancements in both the strength of the interplanetary magnetic field and the solar wind dynamic pressure pushed the magnetopause and bow shock inside of geosynchronous orbit. We present here observations of the unshocked solar wind observed with the Los Alamos magnetospheric plasma analyzer on geosynchronous satellite 1994‐084 near local noon. The magnetosheath was observed intermittently at geosynchronous satellites 1991‐080, 1994‐084, and LANL‐01A, across much of the dayside magnetosphere. Comparisons with theory of the observed bow shock and magnetopause locations at geosynchronous orbit near noon are in reasonable agreement. Using nearly simultaneous observations of the magnetopause and bow shock locations at 0451 UT, we have tested models for the thickness of the magnetosheath and flaring of the magnetopause at that time. Using the least flared of the magnetopause models, the estimated thickness of the magnetosheath would be thicker than would be predicted by either the gas dynamic solution of Spreiter et al. [1966] or the MHD simulations of Cairns and Lyon [1996]. Alternatively, placing the location of the magnetopause at the nose of the magnetosphere in conformance with the thickness of the magnetosheath from Cairns and Lyon [1996] requires less flaring on the flanks of the magnetosphere than currently present in the empirical models. The models also have difficulty predicting the other observed magnetopause crossings far from local noon apparently due to excessive flaring of the models for the conditions on this day.

A comprehensive survey of plasmasphere refilling at geosynchronous orbit

Plasmasphere refilling is one of the most fundamental examples of ionosphere‐magnetosphere coupling. Geosynchronous low‐energy ion measurements are particularly useful in addressing this issue. In this survey we collect almost 11 satellite years of data, from January 1990 to September 2000, from Los Alamos magnetospheric plasma analyzers on board five satellites. The data are sorted according to the level of magnetospheric activity, local time, season, and solar cycle phase for a comprehensive examination of the early‐time refilling rate. These statistical results indicate that the early‐time refilling rate increases with increasing geomagnetic activity. Additionally, the early‐time refilling rate at low solar activity is about a factor of 1.5 – 2 higher than that at high solar activity. This may be related to the higher ionospheric neutral hydrogen density at solar minimum. Moreover, when the geomagnetic activity index is higher than 3, the early‐time refilling rate is observed to be slightly higher during the seasonal equinox periods than at solstice, but no seasonal variation is found at low geomagnetic activity. The late‐time refilling rate is also examined during intervals of prolonged geomagnetic quiet. The refilling process is not linear as a function of time: The early‐time refilling rate ranges between 2.5 and 6.5 cm−3 d−1, while the late‐time refilling rate is found to be ∼ 10 – 25 cm−3 d−1. The late‐time refilling rate is also anticorrelated with solar activity. The plasmasphere at geosynchronous orbit is observed to reach a maximum density at ∼ 100 – 200 cm−3.

Plasma sheet access to the inner magnetosphere

We present here plasma data from the Polar HYDRA instrument giving comprehensive coverage of the inner magnetospheric region from L ∼ 2 outward. Data is projected to an equatorial reference plane yielding a global view of the inner extend of the plasma sheet. We determine the inner boundary for plasma sheet electrons and ions in the μ range 0.05 – 50 eV nT−1 and we compare these to the predicted Alfvén boundaries as a function of the geomagnetic activity index Kp. In general, the simple conventional drift paradigm is shown to be globally consistent with the averaged data in the inner magnetosphere, with electrons adhering better to the predicted boundaries than ions. The data are further compared to the geosynchronous slice as measured by the Los Alamos Magnetospheric Plasma Analyzer (MPA) which measures the crossing point of the Alfvén boundaries at geosynchronous altitudes with much better statistical resolution than Polar. Integral to the drift model used is an assumption about the form of the global electric field. The agreement with data validates the simple corotation and convection electric field used and shows that this model describes well the average transport for a wide range of geomagnetic activity and over a large part of the inner magnetosphere.

Plasmaspheric material at the reconnecting magnetopause

During geomagnetic storms, cold and dense plasmaspheric material is observed to drain toward the dayside magnetopause when the solar wind pressure is strong and the interplanetary magnetic field (IMF) is southward. What is the fate of draining plasmaspheric material at the magnetopause? Does the plasmaspheric material participate in the dayside reconnection and then convect on open field lines through the polar cap? Or does the material become captured into the low‐latitude boundary layer and then convect on closed field lines around the flanks of the magnetosphere? In this paper, we present observations from the Los Alamos magnetospheric plasma analyzers (MPA) onboard five satellites at geosynchronous orbit during 86 plasmaspheric drainage events. For a set of events where cold plasmaspheric material is observed immediately adjacent to the magnetopause/low‐latitude boundary layer, we examine the detailed ion distributions, from ∼1 eV to ∼40 keV, for evidence that the draining plasmaspheric ions and the entering magnetosheath ions are simultaneously present on the same flux tube. Ten cases out of 57 are found where magnetosheath ions and plasmaspheric ions were unambiguously present simultaneously in the same flux tube, which is a signature that the plasmaspheric flux tubes do experience dayside reconnection. An additional ten cases strongly, but not as definitively, support this conclusion. Further, six of seven events with available IMF information have velocity space signatures that are consistent with expectations based on the reconnection process.

Survey of pancake‐shaped warm ion distributions at geosynchronous orbit

It has been proposed that the electromagnetic proton cyclotron instability is a strong source of heating for the anisotropic warm ions observed at geosynchronous orbit. We present here the results of a statistical study of pancake‐shaped warm ion distributions using a 1‐year interval of data observed with the Los Alamos magnetospheric plasma analyzer on the geosynchronous satellite 1994–084. Our results support previous findings that pancake‐shaped warm ion distributions occur more frequently on the dayside of the magnetosphere and occur very close to the plasmapause. We also confirm that the electromagnetic proton cyclotron instability is operating and is constraining the hot proton temperature anisotropy. However, our results indicate that the pancake‐shaped warm ion distributions observed at geosynchronous orbit are probably not generated by this instability but must be due to a different mechanism, possibly heating by lower hybrid waves.

Measurements of early and late time plasmasphere refilling as observed from geosynchronous orbit

Measurements of the cold ion density at geosynchronous orbit obtained by Los Alamos magnetospheric plasma analyzers over the course of 7 years are used to estimate the rate of plasmaspheric refilling at early times in the refilling process (≤24 hours) and at late times (up to several days during intervals of prolonged geomagnetic quiet). While the measured refilling rates are highly variable, we find that plasmasphere refilling at geosynchronous orbit occurs as a two‐step process: for early times the refilling rate is ∼0.6–12 cm−3 d−1, while for later times the refilling rate rises up to 10–50 cm−3 d−1. These results are consistent with a model of plasmasphere refilling which uses Coulomb collisions as a dominant trapping mechanism [see Wilson et al. 1992].

Global modeling of the plasmasphere following storm sudden commencements

We examine the dynamics of the outer plasmasphere during 10 post‐ssc events by comparing observations of cold, dense ions from Los Alamos magnetospheric plasma analyzers on board three widely spaced geosynchronous satellites with output from the Magnetospheric Specification and Forecast Model (MSFM). The MSFM is a data‐driven, operational space weather specification and forecast code originally designed to facilitate U.S. Air Force spacecraft operations. For this study we modified the MSFM to include a cold plasmaspheric ion population that was subject to the effects of ionospheric refilling. We utilized the electron density model of Carpenter and Anderson [1992] and the assumption of charge neutrality to initialize the plasmaspheric proton density within a specified plasmapause. This configuration was then allowed to evolve under the effects of E × B drift and refilling. The modified MSFM clearly shows the development and westward transport of duskside plasmaspheric plumes/tails during periods of enhanced convection and the eastward transport of these structures during decreasing activity. We present a detailed comparison between the data and the model output for one case and a “statistical” analysis of the comparison for all 10 cases. We also compare the model results with previously published observations of plasmapsheric ions and models of plasmaspheric dynamics. The MSFM was able to systematically reproduce the geosynchronous observations with good accuracy in both local time placement and density level. We find the modified MSFM to be an improvement upon previous plasmaspheric models and a useful tool in the interpretation of spatially and temporally separated geosynchronous observations.

Premidnight plasmaspheric “plumes”

To explain observations of brief intervals of cold, dense plasma by geosynchronous satellites in the midnight sector prior to or during substorm onset, it has recently been proposed that dense plasmaspheric plasma is drawn out to geosynchronous orbit in the premidnight region by inductive electric fields during the growth phase of a geomagnetic substorm. We present here the results of a statistical study of such intervals observed with the Los Alamos magnetospheric plasma analyzer (MPA) on geosynchronous satellite 1989‐046 between March and December 1993. We find that these premidnight cold plasma intervals occur only after extended periods of low magnetospheric activity identified by Kp and the midnight boundary index (MBI). We also find that the satellite typically enters the cold plasma region from the trough region and exits it into the plasma sheet. Finally, while measurements of the flow velocity of the cold plasma are rendered uncertain by the asymmetric spacecraft charge or photoelectron sheath, such measurements show no evidence of the outward flow that would be expected from the extrusion hypothesis. Rather, there are some indications that these cold plasma regions flow sunward in the (corotation) satellite frame. These results suggest an alternative explanation for the premidnight cold plasma: corotation‐dominated transport of day side plasmaspheric structures into the premidnight sector. The implications of our observations for the extrusion hypothesis and for the alternative explanation are discussed.

Warm protons at geosynchronous orbit

Ions in the outer magnetosphere typically are observed to consist of a hot (T ∼ 10 keV) component and a cool constituent with energies of a few eV. The hot component often is observed to be anisotropic with T⊥ > T‖, where the subscripts denote directions relative to the background magnetic field, whereas the cool ions exhibit a wide variety of anisotropies. The Los Alamos magnetospheric plasma analyzer (MPA) instruments measure ion and electron velocity distributions from about 1 eV to about 40 keV at geosynchronous orbit. In the afternoon and evening sectors MPA observations show that cool ions, which are assumed to be protons, are sometimes “warm” with temperatures of order 10 eV and T⊥ > T‖. Theory and simulations of the electromagnetic proton cyclotron instability, which is driven by the hot proton anisotropy, have shown that the resulting enhanced field fluctuations heat initially cool protons. Moreover, this process implies a scaling relation for the apparent temperature of the warm protons as a function of the relative densities of the two components. This manuscript describes an examination of MPA data which shows that this scaling relation provides an approximate upper bound for a selected set of warm proton temperatures observed at geosynchronous orbit. This result is consistent with the hypothesis that the proton cyclotron instability is an important energization source for warm protons at geosynchronous orbit.

Electromagnetic proton cyclotron instability: Interactions with magnetospheric protons

The temperature anisotropy of the hot (keV) protons of the outer magnetosphere can excite the electromagnetic proton cyclotron instability. Wave‐particle scattering by this instability not only maintains the hot proton anisotropy at or below an upper bound but also imparts energy to the cool (eV) proton component. This instability and its consequences are examined through the use of linear and second‐order Vlasov theory and one‐dimensional hybrid simulations in a homogeneous plasma model which represents these two protonic constituents. A different form for the upper bound on the hot proton anisotropy is derived from linear theory and the simulations; comparison against plasma observations from Los Alamos magnetospheric plasma analyzers in geosynchronous orbit shows very good agreement. Second‐order theory and simulations are used to obtain a different scaling for the late‐time apparent temperature of the cool proton component Tc which indicates that it is inversely correlated with the cool proton density nc. Observations in both the plasmasphere and the outer magnetosphere sometimes show a similar inverse correlation, suggesting that the electromagnetic proton cyclotron instability is an important contributor to cool proton heating in these regions.

Hot proton anisotropies and cool proton temperatures in the outer magnetosphere

The hot protons of the outer magnetosphere typically exhibit a temperature anisotropy such that T⊥/T∥ > 1, where perpendicular and parallel symbols denote directions relative to the background magnetic field. If this anisotropy is sufficiently large, the electromagnetic proton cyclotron anisotropy instability may be excited. This instability is studied using linear Vlasov theory and one‐dimensional hybrid simulations for a homogeneous plasma model representative of conditions in the outer magnetosphere with a hot anisotropic proton component (denoted by subscript h) and a cool, initially isotropic proton component (subscript c). Linear theory yields an instability threshold condition on the hot proton temperature anisotropy whereas the simulations imply an upper bound on T⊥h/T∥h; both the threshold and the upper bound have similar scalings with the maximum growth rate γm, the parallel β of the hot component, β∥h, and the relative density of the hot component nh/ne. An analysis of plasma observations from the Los Alamos magnetospheric plasma analyzer (MPA) in geosynchronous orbit finds that the maximum value of the hot proton temperature anisotropy approximately satisfies the predicted scalings with β∥h and nh/ne and yields the proportionality factor that quantifies this upper bound. The simulations are also used to examine the heating of the cool protons by the proton cyclotron instability. The simulations yield a scaling for the dimensionless late‐time cool proton average temperature Tc/T∥h as (nh/ne)/β∥h0.5. Analysis of MPA data shows that the observed values of Tc/T∥h have similar scalings and again yield the proportionality factor which quantifies this relationship.