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Laboratory for Atmospheric and Space Physics


SAMPEX science investigations are addressing a very broad range of questions, including:

Cosmic Rays: Ionization states, energy spectra, isotopic composition, of galactic and anomalous cosmic rays.
Solar Energetic Particles: Isotopic composition, impulsive event spectra, ionization states in large SEP events. SEPS with anomalously low ionization states.
Magnetospheric Physics: Acceleration mechanisms, global surveys, precipitation bursts of relativistic electrons, and protons. Space weather.
Magnetosphere-Atmosphere Link: Energy input from precipitating electrons influencing middle atmosphere NOy which affect global ozone levels.

Cosmic Ray Studies

One of the significant achievements of SAMPEX has been to confirm the existence of the trapped component of the anomalous cosmic rays – ACR’s. During its first year SAMPEX has confirmed that ACRs are singly charged, and it has located a narrow belt of trapped ACRs within the inner of Van Allen radiation belts. SAMPEX found the third radiation belt to be composed of oxygen with smaller amounts of N and Ne and very little C. The observed relative abundances of C to N to O are consistent with those of ACRs. The striking near-absence of C in this third radiation belt is inconsistent with other possible sources of trapped particles such as the solar wind, SEPS, or GCR’s. The locations of oxygen events (trapped ACRs) projected onto world map is shown.

The omnidirectional intensity of trapped oxygen is shown below in the R-Lambda polar coordinate system. The solid circle represents the approximate R value of the Earth’s upper atmosphere in the South Atlantic region above which the data were taken. The dashed circle represents the approximate R value (1.3) of SAMPEX in this region. Intensities outside the dashed circle calculated assuming that the intensities perpendicular to the magnetic field are the same as those observed at the dashed circle. Right side figure shows intensities calculated assuming that there are no trapped particles with mirror points in this region.

Solar Energetic Particle Studies

Solar energetic particle ionization states
The ionization states of heavy ions measured in a number of large solar energetic particle (SEP) events have been found to be roughly consistent with equilibrium temperatures of a few x 10e+6 K, but with significant element to element differences. SEP charge states have provided some of the strongest evidence that in large (“gradual”) flares the energetic particle seed population is the corona, rather than the much hotter (>10e+7 K) site of the optical flare.

Instruments measuring solar particle charge states in inter-planetary space using electrostatic deflection techniques (both prior to SAMPEX and on the upcoming ACE mission) are limited to the energy range near 1 MeV/nucleon. On SAMPEX, however, we have used the geomagnetic field to differentiate charge states, with a vastly enlarged energy range available for study. This makes it possible to extend charge state measurements to considerably higher energies, thereby probing important new aspects of the acceleration process and the nature of the acceleration regions.

This figure shows deduced charge states as a function of energy for O and Fe measured by LICA, HILT, and MAST during the Oct/Nov 1992 SEP event. Note the energy dependence in the mean iron charge state.

Heavy ion spectra from LICA and HILT.

Since the launch of SAMPEX in July 1992 through the present time (October 1997) , only the solar particle events of Oct/Nov 1992 have been large enough to make good use of this approach. For most of the observed elements, the mean charge states determined by SAMPEX are consistent at all energies with those measured directly in previous particle events around 1 MeV/nuc, with the largest exception being for iron. The LICA results, averaged over the energy range ~0.5-5 MeV/nuc, show a mean charge state of 11.04±0.22 for Fe, while the average from Luhn is 14.09±0.09. The mean value rises to 15.18±0.73 over the ~28-65 MeV/nuc energy interval of MAST, with some indication of an increase even across this interval. Although the cause of this energy dependence is not fully understood, the effect may be used to place limits on the particles’ residence time in coronal loops.

Research by SAMPEX team has concentrated on 3He-rich impulsive events and energetic particle ionization states. Novel instrumentation on SAMPEX has discovered new properties that add to the constraints on models for the origin and acceleration of SEPS.

Solar particle events with large enrichments of 3He are unusual not only because of large 3He enrichment, but also have because of heavy ion and electron enrichment, high ionization states, lack of association with coronal shocks or mass ejections … etc. Data from a number of 3He events have resulted in spectra for several elements show in the figure above and to the left.
Ref: The Ionic charge of Solar Energetic Particles with Energies of 0.3-70 MeV/nucleon – By M. Oetliker et. al.
The high energy end of the spectra have been used to constrain theoretical explanations for 3HE-rich SEP acceleration.

Magnetospheric Studies

SAMPEX instrumentation has given new insights into the behaviour of energetic electrons in the Earth’s radiation belts. The HILT, PET, and LICA sensors provide an unprecedented combination of sensitivity and time resolution. The SAMPEX nearly polar orbit and its short period provides many complete latitudinal and thus L value samplings per day. Areas in which SAMPEX instruments provide important and unique information include:

  • Long-term observations
  • Energy input into the Middle atmosphere
  • Magnetosphere-Middle atmosphere coupling
  • High time resolution measurements

Trapped Magnetospheric Electrons, long term observations :

The sources, redistribution processes, and loss mechanisms of radiation belt particles are primary issues in magnetospheric physics. High energy electrons hold special interest because of their ubiquity in the Earth’s magnetosphere and their importance to other scientific and technological issues. These include dose and bulk charging effects in space vehicles in most Earth orbits, coupling to the Earth’s middle atmosphere, and the understanding of acceleration processes in remote astrophysical objects.

A comprehensive view is provided in figure to the right, which shows the PET >0.4 MeV counting rate, sorted according to L value versus time. The logarithm of the electron intensity level in each L bin has been color coded according to the reference bar at the right of the figure for all L values between 1.0 and 8.0. Numerous abrupt flux increases occurred in the outer zone (L > 2.5) throughout the multi-year period shown. Abrupt enhancements occur within a day, extend over a broad range in L and reach to high latitudes, of-ten beyond L =3D 6 (geostationary orbit). The >1 MeV and the >3 MeV channels from HILT exhibit the same basic features. Even the slot region around L ~ 2.5, which separates the inner and outer zones, is often temporarily filled with electrons. Previous results suggested that the slot regions normally filled only during major geomagnetic storms. The SAMPEX data show more frequent filling.

SAMPEX data show the remarkable variation in electron properties as one goes to the heart of the outer radiation belt at L=4. These multi MeV electron fluxes vary by factors of 10-100 on time scales of a day. The fluxes correlate with high speed solar wind streams, peaking a day or two after the solar wind speed peaks upstream of the magnetosphere. The maximum correlation between the high speed streams and radiation belt fluxes occurs with an increasing time delay for higher energies and lower L values.

HILT data from 15 May 1993 to 1 February 1994 (Days 500-763 of 1992).

In the figure to the right, it is evident that the outer zone electrons exhibit very prominent flux peaks around Days 600, 680, and 750. These relativistic electron enhancements were associated with major spacecraft operational problems at geostationary orbit on January 20, 1994 including the loss of attitude control on Intelsat-K, and failure of the momentum wheel controls on the Anik E1 and E2 spacecraft.

High energy electron acceleration:

This study suggests that the magnetosphere is frequently being driven very hard by recurrent solar wind streams. Figure to the right shows electron fluxes measured by SAMPEX P1 channel (>~).4 Mev) in PET from July of 1992 to July 1993, in bins of 0.1 L-values and range 1 1 MeV fluxes from the HILT instrument are not shown.

Count rate channels with electron energy thresholds ranging from 0.4 MeV to 3.5 MeV in three different instruments have been used to examine relativistic electron variations as a function of L-shell parameter and time. A long run of essentially continuous data(July 1992 – July 1993) show substantial acceleration of electrons throughout much of the magnetosphere on rapid time scales. This acceleration appears to be due to solar wind velocity enhancements and is surprisingly large in that the radiation belt “slot” region often is filled temporarily and electron fluxes are strongly enhanced even at very low L-values (L | 2). A superposed epoch analysis shows that electron fluxes rise rapidly for 2.5 |< L |< 5. These increases occur on a time scale of order 1-2 days and are most abrupt for L values near 3. The temporal decay rate of the fluxes is dependent on energy and L value and may be described by J=Ke-t/t0 with t0 | 5-10 days. Thus, these results suggest that the Earth’s magnetosphere is a cosmic electron accelerator of substantial strength and efficiency.

It can be seen that there are numerous abrupt flux increases over a wide range of L values. Event the slot region around L ~2.5, which separates the inner and outer zones, is often temporarily filled with electrons. This is at variance with previous suggestions that such filling occurs only during major storms.

Electron flux enhancements observed by SAMPEX instruments have also been found correlate with some recent satellite failures (ANIK E-1, and 2), lending credence to the hypothesis of deep-dielectric charging in spacecraft leading to their failure.

Coupling of magnetospheric precipitating electrons to the middle atmosphere

This figure shows altitude-time variation of daily NOy formation expressed as a percentage of an average reference NO profile observed by HALOE. The figure illustrates both the rapid and longer-term variations of the NOy formation rate.

In the area of Earth sciences, there has been a long-term effort to understand the natural variation of the middle atmospheric chemical species important to the maintenance of global O3. Among these species, the oxides of nitrogen (NOy) are particularly important since they lead to the catalytic destruction of O3. These natural variations must be understood in order to unambiguously assess the effect of man’s activities on O3. Such knowledge is critical since O3 shields Earth from harmful UV radiation and is crucial in establishing the thermal structure of the stratosphere and, hence, its dynamic climatology.

Of particular interest is the effect of solar activity on this natural variability. It is well known that variations in the Sun’s UV flux modulate stratospheric O3. It has been suggested that stratospheric O3 may also be affected by the formation of mesospheric NO due to precipitating energetic electrons followed by advective transport of the NO into the stratosphere (during the late fall, winter, and spring) where it may enhance the catalytic destruction of O3.

Several studies have shown that relativistic electron precipitation events can, through energy deposition, provide the dominant ionizing process between 50 and 120 km. It is known that such ionizing processes can lead to the formation of oxides of nitrogen (NOy) in the middle atmosphere. It is shown as a percentage of the average of many NO pro files (between 45 and 65=B0 latitude) ob-served by the UARS Halogen Occultation Experiment (HALOE) for 25 < Z < 130 km.

Crucial to a firm resolution of this issue are the synergistic observations of energetic electrons from low Earth orbit together with NO observations by HALOE through solar maximum. HALOE is planned to continue operating indefinitely. SAMPEX observations through solar maximum will allow a quantitative evaluation of this potentially important,highly interdisciplinary, and little recognized solar terrestrial coupling mechanism. Confirmation of this coupling would be seminal since it would open the door to many interesting interdisciplinary studies linking solar, space, and atmospheric physics.