Imager For Magnetopause-to-Aurora Global Exploration Research Articles

Interplanetary magnetic field control of afternoon‐sector detached proton auroral arcs

Data from the Far Ultraviolet Imager (FUV) on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite show that subauroral proton arcs appear in the afternoon sector during geomagnetically disturbed periods when the interplanetary magnetic field rotates either from south to north or from west to east and when the magnetosphere is moderately compressed. Time series of proton aurora images show that the proton emissions are generally aligned along the equatorward part of the auroral oval. However, when interplanetary magnetic field (IMF) Bz changes from negative to positive the auroral oval contracts toward higher latitudes while the ring current proton precipitation remains stationary, resulting in a separation of several degrees between the latitude of the new oval position and a subauroral proton arc in the afternoon sector. A similar effect occurs when IMF By rotates from negative to positive, in which case the oval in the afternoon sector retreats toward higher latitudes, again leaving a separation between the oval and the subauroral proton arc of several degrees. Comparisons with low‐altitude and geosynchronous satellite data show that the subauroral proton arc is caused by the precipitation of protons with energies from several keV to 30 keV and is likely associated with the existence of a plasmaspheric “drainage plume.” In contrast, the proton emissions along the main oval are caused by protons with energies generally less than 10 keV.

Cusp aurora dependence on interplanetary magnetic field Bz

The Imager for Magnetopause to Aurora Global Exploration (IMAGE) Spectrographic Imager (SI12) detects Doppler‐shifted Lyman alpha emissions created by charge exchange and de‐excitation of precipitating protons in the atmosphere. At high latitudes near local noon, emissions consistent with the location of the cusp foot point have distinct interplanetary magnetic field (IMF) dependence. For northward IMF a spot of emissions is observed poleward of the dayside auroral oval. By tracing the magnetic field lines from this spot using a model magnetosphere it is shown that the cusp foot point maps to a narrow region on the high‐latitude magnetopause where antiparallel magnetic reconnection may be occurring. As the IMF turns southward, the spot merges into the auroral oval, producing a broad region of intense emissions centered near local noon. By tracing the magnetic field lines from this broad region using a model magnetosphere it is shown that the cusp foot point maps to a relatively broad region on the dayside magnetopause where component magnetic reconnection may be occurring.

Quantitative test of the cavity resonance explanation of plasmaspheric Pi2 frequencies

A Pi2 wave event was observed by the CRRES satellite on day 48 (17 February) of 1991, 1220–1250 UT [Takahashi et al., 2001]. Using the electron density profile measured by CRRES on its outward pass (during which the wave event was observed), and assuming that the ions are all H+, we calculated the theoretical plasmaspheric cavity resonance frequencies using a two‐dimensional MHD simulation in dipole geometry (azimuthal symmetry). Our theoretical frequencies are in good agreement with the observed frequencies, and this supports the cavity resonance mechanism as an explanation for the observations. The observed and theoretical harmonic frequency ratios f2/f1 and f3/f1 were in rough agreement with earlier studies. Comparison of the outbound and inbound legs of the CRRES orbit on 17 February 1991 shows that the density on the outbound leg at magnetic local time (MLT) = 20–23.4 was different than that on the inbound leg at MLT = 23.4–4.5. On the outbound leg an enhanced density region beyond the region of the nominal plasmapause (which may have been part of a plasmaspheric tail [Sandel et al., 2001]) played an important role in determining the cavity resonance frequencies. Observations by the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft have revealed that there is often a great deal of azimuthal structure in the plasmasphere (including such tails). Such structure may play an important role in determining observed Pi2 frequencies.

Proton aurora in the cusp

Frequently, the Far Ultraviolet Instrument (FUV) on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft observes intense ultraviolet emission from a localized dayside region poleward of the general auroral oval location. This emission is especially distinct in the Doppler‐shifted emission of hydrogen atoms produced by precipitating protons. We interpret this as a direct signature of proton precipitation into the cusp after reconnection of magnetospheric lobe field lines. This cusp signature appears only when the interplanetary magnetic field (IMF) has a positive northward Bz component. However, the intensity of the precipitation, and hence the intensity of UV emission, is not controlled by the magnitude of Bz but rather by the solar wind dynamic pressure. A statistical analysis of 18 cases observed in summer and fall 2000 shows good correlation between the UV intensity and the dynamic pressure and between the location in local time and the IMF By component. A quantitative analysis of observations from all three FUV subinstruments allows for an estimate of proton and electron energy fluxes during these times. In general, these estimates agree with results from in situ measurements by spacecraft and show that during these times, protons may contribute significantly to the overall energy deposition into the cusp.

From nanometers to gigaparsecs: The role of nanostructures in unraveling the mysteries of the cosmos

High launch costs and the extreme distance to astrophysical objects place a premium on astrophysical instrumentation with the highest attainable sensitivity and resolution at the lowest possible weight and cost. Many interesting and useful optical phenomena occur when the size and placement accuracy of features are comparable to, or smaller than, the wavelength of light. These considerations have compelled us to develop a variety of nanotechnoligies that have now been utilized in space physics instrumentation on nine missions. These include 200- and 400-nm-period membrane-supported transmission gratings for high-resolution spectroscopy of astrophysical x-ray sources, mesh-supported transmission gratings for solar extreme ultraviolet (EUV) monitoring, and UV nanofilters with 45 nm slots that are key components of atom cameras observing Earth’s magnetosphere. This article will describe instruments on space missions where we have applied nanotechnology. One application is the NASA Chandra Observatory x-ray telescope, for which we manufactured a large quantity of transmission gratings for high-resolution spectroscopy. Chandra is now returning a torrent of high-quality x-ray images and spectra from such interesting objects as supernova remnants, the accretion disks around black holes and neutron stars, stellar coronae, galaxy cluster cooling flows, and other x-ray-emitting objects up to gigaparsecs distant. [A short astronomy lesson: As Earth orbits Sol, nearby stars in the sky appear to wobble due to parallax. At a distance of one parsec (a “parallax-second”), the diameter of Earth’s orbit (∼1.5×1011 m) subtends one arcsecond, so a parsec is around 3.3 light years, or 3.1×1016 m. For reference, the nearest star is around a parsec away, our Milky Way galaxy is a few kiloparsecs across, nearby galaxies are megaparsecs away, and the known universe is measured in gigaparsecs (1025 m).] Another application is the atom “camera” on the NASA Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft that studies Earth’s magnetosphere, the belt of plasma around the Earth formed by swept-up ions from the Solar wind trapped in the bottle of Earth’s magnetic field. The camera images the magnetosphere in the “light” of neutral atoms, rather than photons, emitted from the plasma due to charge exchange processes. We developed nanofilters, consisting of 500-nm-thick gold foils with 45-nm-wide slots, that are designed to block unwanted deep-UV and EUV photons which would otherwise overwhelm the detector with a million-to-one noise-to-signal ratio, thus allowing the camera to detect the weak atom fluxes. IMAGE is now sending back spectacular atom movies of the magnetosphere revealing a wealth of new information about this complex and dynamic environment. Finally, I describe work in our laboratory aimed at developing microtechnology for the shaping and assembly of glass microsheet optics to few-nanometer accuracy. We believe these new x-ray optics will spawn a new generation of diffraction-limited x-ray telescopes with massive collecting areas and resolution approaching 0.1 microarcsecond (∼1 picoradian). These new telescopes may enable the direct imaging of the massive black holes believed to lurk at the center of most galaxies.

Observations of neutral atoms from the solar wind

We report observations of neutral atoms from the solar wind in the Earth's vicinity with the low‐energy neutral atom (LENA) imager on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft. This instrument was designed to be capable of looking at and in the direction of the Sun. Enhancements in the hydrogen count rate in the solar direction are not correlated with either solar ultraviolet emission or suprathermal ions and are deduced to be due to neutral particles from the solar wind. LENA observes these particles from the direction closest to that of the Sun even when the Sun is not directly in LENA'S 90° field of view. Simulations show that these neutrals are the result of solar wind ions charge exchanging with exospheric neutral hydrogen atoms in the postshock flow of the solar wind in the magnetosheath. Their energy is inferred to exceed 300 eV, consistent with solar wind energies, based on simulation results and on the observation of oxygen ions, sputtered from the conversion surface in the time‐of‐flight spectra. In addition, the sputtered oxygen abundance tracks the solar wind speed, even when IMAGE is deep inside the magnetosphere. These results show that low‐energy neutral atom imaging provides the capability to directly monitor the solar wind flow in the magnetosheath from inside the magnetosphere because there is a continuous and significant flux of neutral atoms originating from the solar wind that permeates the magnetosphere.

Whistler-mode wave-injection experiments in the plasmasphere with a radio sounder

Whistler-mode wave-injection experiments with a high-altitude radio sounder offer an opportunity to greatly extend the observing power of satellites such as imager for magnetopause-to-aurora global exploration (IMAGE) when the satellite is within or near the plasmasphere or at low altitudes over the polar regions. We use as an example the radio plasma imager (RPI) instrument on IMAGE, which includes crossed 500-m electric antennas in the spin plane and a 20-m antenna along the spin axis (for reception only). The 500-m antennas approach a half-wavelength at whistler-mode frequencies in the 3– 30 kHz range and should have a radiation efficiency of 1–10%. The wave power within ∼100 km of the transmitter should be greater than that produced by wave injection from the ground-based very low-frequency (VLF) transmitter at Siple, Antarctica, thus making possible experiments on wave–particle energy and momentum exchange. We use ray tracings along a sample IMAGE orbit (polar, apogee ∼8 R E) to show the conditions under which transmitted signals may return to the satellite as echoes, following reflection, or be observed by satellites of opportunity, such as EXOS-D. We find that ground reception of transmitted signals should be possible as a result of a linear mode conversion process in regions of ionospheric irregularities. We discuss the determination of wave-normal angles of returning signals, which will aid in identifying the signal path and obtaining information on plasma boundaries and irregularities. The science topics that may be addressed include: (1) investigation of the nonlinear process by which weak coherent waves excite VLF emissions; (2) probing plasmaspheric density structure, including plasmaspheric density cavities, field aligned waveducts, density irregularities in the plasmapause region, and the ionospheric density structure where conversion of whistler-mode wave energy to quasi-electrostatic lower hybrid (LHR) waves (and vice versa) can take place.

Mission allows magnetospheric physicists to “See” the Invisible

Since the launch of Explorer 1 in 1958, magnetospheric physicists have studied regions they could not “see” but could visualize only with the aid of conceptual models that rely on the statistical analysis of measurements made at single points in space at different points in time. Such measurements yielded “pixels” of information that were painstakingly and methodically developed into the mental pictures that reveal the structure and dynamics of our geospace environment. However, with the launch of the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft on March 25, 2000, the magnetospheric physics community has at long last acquired the ability—possessed for many years by the solar physics and astrophysics communities— to see the objects of its inquiry: the plasmas that populate the inner magnetosphere.

First results from the Radio Plasma Imager on IMAGE

The Radio Plasma Imager (RPI) is a 3 kHz to 3 MHz radio sounder, incorporating modern digital processing techniques and long electronically‐tuned antennas, that is flown to large radial distances into the high‐latitude magnetosphere on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite. Clear echoes, similar to those observed by ionospheric topside sounders, are routinely observed from the polar‐cap ionosphere by RPI even when IMAGE is located at geocentric distances up to approximately 5 Earth radii. Using an inversion technique, these echoes have been used to determine electron‐density distributions from the polar‐cap ionosphere to the location of the IMAGE satellite. Typical echoes from the plasmapause boundary, observed from outside the plasmasphere, are of a diffuse nature indicating persistently irregular structure. Echoes attributed to the cusp and the magnetopause have also been identified, those from the cusp have been identified more often and with greater confidence.

Global dynamics of the plasmasphere and ring current during magnetic storms

Simultaneous global images of the plasmasphere and the ring current were obtained with the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft during the geomagnetic storms of 24 May 2000 and 29 July 2000. The plasmasphere images confirm the theoretically predicted development of long plasma tails in the dusk hemisphere during magnetic storms. They also reveal several unexpected structures, including a sharp azimuthal gradient or shoulder, which forms on the night side and corotates with the Earth for many hours, and a narrow ion trough, which is located well inside the main plasmapause in the late evening sector. The ring current images show the peak of the ring current to be nearly centered on the plasmapause, which is consistent with ring current decay models that include coulomb scattering and wave‐particle interactions as well as charge exchange.

Modeling ring current proton precipitation by electromagnetic ion cyclotron waves during the May 14–16, 1997, storm

We study mechanisms contributing to proton precipitation from the ring current during the May 14–16, 1997, geomagnetic storm. This storm was caused partly by Bz< 0 fields in the sheath region behind an interplanetary shock and partly by the magnetic cloud driving the shock. The storm was characterized by a maximum Kp=7− and a minimum Dst=−115 nT and had a distinctive two‐phase decay related to the passage of the ejection at the Earth. We model the ring current development caused by adiabatic drifts and losses due to charge exchange, Coulomb collisions, wave‐particle interactions, and atmospheric collisions at low altitudes. The nightside magnetospheric inflow is simulated using geosynchronous Los Alamos National Laboratory data, whereas the dayside free outflow corresponds to losses through the dayside magnetopause. We calculate the equatorial growth rate of electromagnetic ion cyclotron waves with frequencies between the oxygen and helium gyrofrequencies and their integrated wave gain as the storm progresses. The regions of maximum wave amplification compare reasonably well to satellite observations. A time‐dependent global wave model is constructed, and the spatial and temporal evolution of precipitating proton fluxes during different storm phases is determined. We find that the global patterns of proton precipitation are very dynamic: located at larger L shells during prestorm conditions, moving to lower L shells as geomagnetic activity increases during storm main phase, and receding back toward larger L shells with storm recovery. However, the most intense fluxes are observed along the duskside plasmapause during the main and early recovery phase of the storm and are caused by plasma wave scattering. This study is relevant to the analysis of the anticipated new data sets from the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) and Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) missions.

The Image Science and Mission Operations Center

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) will produce forefront science by quantifying the response of the magnetosphere to the time variable solar wind. It will acquire, for the first time, a variety of three-dimensional images of magnetospheric boundaries and plasma distributions extending from the magnetopause to the inner plasmasphere. The images will be produced on time scales needed to answer important questions about the interactions of the solar wind and the magnetosphere. The IMAGE team will provide open access to all IMAGE data. Thus there will be no proprietary rights or periods. All IMAGE data products will be archived and available to the scientific research community. The IMAGE mission will operate with a near 100% duty cycle with all instruments in their baseline operational modes. A Science and Mission Operations Control Center or SMOC has been developed at the NASA Goddard Space Flight Center (GSFC) to be the main data and command processing system for IMAGE. The IMAGE Level-0 data will be processed into Level 0.5 and Level-1 data and browse products within 24 hours after their receipt of raw data in the SMOC. These data products will be transferred to the NSSDC., for long-term archiving, and posted immediately on the world-wide-web for use by the international scientific community and the public.

Image Mission Overview

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission is the first mission in NASA's MIDEX (Mid-size Explorer) program. It is the first satellite mission that is dedicated to imaging the Earth's magnetosphere. IMAGE will utilize the techniques of ultraviolet imaging, neutral atom imaging, and radio plasma imaging to map out global distributions of the electron and proton aurora; the helium ions of the plasmasphere; the ionospheric ion outflow; the medium-energy ions of the near-Earth plasma sheet, ring current, and polar cusp; the high-energy ions of the ring current and trapped radiation belts; and the total plasma density from the ionosphere out to the magnetopause. The imaging perspective is from an elliptical polar orbit with apogee at latitudes from 40° to 90° in the northern hemisphere. For ultraviolet and neutral atom imaging, the time resolution is set by the two-minute spin period of the IMAGE spacecraft, which will be sufficient to track the development of magnetospheric substorms. An important feature of the IMAGE mission is its completely open data set with no proprietary data or intervals. All data, along with software needed for plotting and analysis, will be available within 24 hours of acquisition.