Small Explorer Spacecraft Research Articles

The Interface Region Imaging Spectrograph (IRIS)

The Interface Region Imaging Spectrograph (IRIS) small explorer spacecraft provides simultaneous spectra and images of the photosphere, chromosphere, transition region, and corona with 0.33-0.4 arcsec spatial resolution, 2 s temporal resolution and 1 km/s velocity resolution over a field-of-view of up to 175 arcsec x 175 arcsec. IRIS was launched into a Sun-synchronous orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a 19-cm UV telescope that feeds a slit-based dual-bandpass imaging spectrograph. IRIS obtains spectra in passbands from 1332-1358, 1389-1407 and 2783-2834 Angstrom including bright spectral lines formed in the chromosphere (Mg II h 2803 Angstrom and Mg II k 2796 Angstrom) and transition region (C II 1334/1335 Angstrom and Si IV 1394/1403 Angstrom). Slit-jaw images in four different passbands (C II 1330, Si IV 1400, Mg II k 2796 and Mg II wing 2830 Angstrom) can be taken simultaneously with spectral rasters that sample regions up to 130 arcsec x 175 arcsec at a variety of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to emission from plasma at temperatures between 5000 K and 10 MK and will advance our understanding of the flow of mass and energy through an interface region, formed by the chromosphere and transition region, between the photosphere and corona. This highly structured and dynamic region not only acts as the conduit of all mass and energy feeding into the corona and solar wind, it also requires an order of magnitude more energy to heat than the corona and solar wind combined. The IRIS investigation includes a strong numerical modeling component based on advanced radiative-MHD codes to facilitate interpretation of observations of this complex region. Approximately eight Gbytes of data (after compression) are acquired by IRIS each day and made available for unrestricted use within a few days of the observation.

Ionospheric erosion by Alfvén waves

Using observations from the FAST small explorer spacecraft, we present fields and plasma observations above the dayside auroral oval showing the erosion of ionospheric plasmas from the topside ionosphere by the action of Alfvén waves. Using interferometric techniques, the waves are shown to approximately obey the expected dispersion for Alfvén waves with transverse scales extending from greater than electron inertial lengths down to ion gyroradii. Measurements of the plasma density where these waves are observed show that over latitudinal widths exceeding 100 km total depletion of the cold ionospheric plasma can occur. These depleted regions or cavities are populated by magnetosheath plasmas, upgoing transversely accelerated ionospheric ions, and downgoing field‐aligned electrons. The ionospheric ions and field‐aligned electrons are distributed as conics and beams, respectively. Poynting flux observations on the density gradients comprising the cavity walls show that these waves are directed downward and focused inward toward regions of lower density. Wave phase velocity measurements, while subject to significant uncertainty, show that the wave vector is directed transversely outward from the cavity. These observations suggest a feedback model for Alfvén wave focusing and ion heating on density gradients that can lead to intense ion outflow from the ionosphere and subsequent depletion of ionospheric plasmas.

Rhessi observations of particle acceleration in solar flares

The NASA Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) Small Explorer spacecraft was launched on 5 February 2002. Its primary objective is to investigate particle acceleration and energy release in solar flares through imaging spectroscopy of flare hard X-ray/gamma-ray continuum and gamma-ray lines emitted by energetic electrons and ions, respectively. Here we present preliminary results on the hard X-ray imaging spectroscopy of a flare, including the spatial variation with energy of the hard X-ray sources, the energy spectra and timing of the individual sources, and spectral features and total energy content. RHESSI observes continuous solar emission in the 3–10 keV energy range, with many microflares from many active regions. These microflares have a non-thermal power-law component similar to normal flares but with much steeper spectral slopes. We also present the first high-resolution gamma-ray line spectrum and the first imaging of gamma-ray lines, from the large X4.8 flare of 23 July 2002.

Imaging ion outflow in the high-latitude magnetosphere using low-energy neutral atoms

The measurement of neutral atom fluxes generated by charge exchange with the Earth's geocorona has recently been shown to provide the capability to image the magnetosphere. We investigate neutral oxygen fluxes produced by charge exchange from the cusp/cleft ion fountain population. Using an empirical cusp/cleft ion fountain model, an empirical variation of the geocoronal neutral hydrogen density with distance, and typical values for charge-exchange cross sections, line-of-sight integrations are performed to calculate the neutral oxygen flux at arbitrary locations in space. The resulting images are evaluated for a set of orbital positions of the proposed HI-LITE small explorer spacecraft. The resulting neutral oxygen fluxes are high enough for imaging with a low-energy neutral atom imaging instrument (ILENA) onboard the spacecraft.