ALBERT

Description: The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) was the first specifically dedicated instrument to observe lightning-induced transient luminous events (TLE) sprites, elves, halos, and gigantic jets from space. The Imager is an intensified CCD system operating in the visible wavelength region with a filter wheel to select from 6 positions with filters. The Imager has a 5°x20° (vertical x horizontal) field of view (FOV). The Spectrophotometer (SP) is populated with 6 photometers with individual filters for emissions from the far ultra-violet to the near-infrared. An Array Photometer with two channels operating in the blue and red provides altitude profiles of the emission over 16 altitude bins each. The Associated Electronics Package (AEP) controls instrument functions and interfaces with the spacecraft. ISUAL was launched May 21, 2004 into a sun-synchronous 890 km orbit on the Formosat-2 satellite and has successfully been collecting data ever since. ISUAL is running on the night side of the orbit and is pointed to the east of the orbit down towards the limb. The instrument runs continuously and writes data to a circular buffer. Whenever the SP detects a sudden signal increase above a preset threshold, a trigger signal is generated that commands the system to keep the data for about 400 msec starting from ~50 msec before the trigger. Over its lifetime of ~11 years the system recorded thousands of TLE and also successfully observed aurora and airglow.

Description: We report the blue luminous events observed by the ISUAL payload onboard the FORMOSAT-2 satellite. The ISUAL 427.8-nm-filtered Imager campaign was conducted near Australia in the summer of the Southern Hemisphere during 2008 Feb-Mar. On February 18, 2008, the 427.8-nm-filtered Imager recorded a series of blue luminous events. One blue starter and nine smaller blue starters were recorded in 2 minutes and 34 seconds in a localized region with the radius 〈 4 km over the cloud top. The average time interval between subsequent blue luminous events was ~ 17 second. The occurrence rate of blue luminous events was 3.5 events per minute, slightly lower than the occurrence rate of pixies (4.2 events per minute) but higher than the occurrence rate of gnomes and blue jets in the previous observations. The recorded first blue starter lasted up to 2–3 frame times (60–90 ms) and extended its altitude about 8 ± 0.3 km with a width of ~2-4 km over the cloud top. After the first blue starter, subsequent 9 smaller blue starters had the decreased heights of ~2-4 km, and their optical duration was shorter and is down to 1 ms. But their major emissions were 2PN 2 and 1NN 2 + , without lightning OI 777.4 nm emission. The ISUAL recorded blue smaller starters had the spatial average brightness of 130 kR for the 427.8 nm-filtered Imager with exposure time (29 ms) and 1.2 MR for the spectrophotometer (337 nm). Using the spectrophotometer, the emission time of blue starters was 1 ms. It is estimated that the 1NN 2 + emission was ~ 22 MR and the 2PN 2 emission was ~ 132 MR. We can estimate the degree of ionization was 10 −11 – 10 −12 in these blue luminous events using the 427.8nm-filtered Imager measured 1NN 2 + (0,1) emission.

Description: The elve is the dominant type of Transient Luminous Event (TLE) at the base of the E-region ionosphere. The hydroxyl nightglow (OH* nightglow hereafter) has been reported at a similar altitude. Statistical studies show that the mean height of elve and OH* nightglow is 87 km and 87.5 km respectively, and that 91% of the 291 limb elves are located within +/- 5 km of the altitude of the brightest OH* nightglow emission and both elves and OH* nightglow show the semiannual oscillation (SAO) at low latitude. The physical reasons for the collocation of elves and OH* nightglow is a main focus of attention. A model for elve emission with the environmental-adapted electron density profile is tested with three density profiles of atomic oxygen (O) : the original profile from the NRLMSISE-00 model and two other profiles which are shifted 5 km upward/downward from the original. For higher altitudes of the given O density profile, the peak altitudes of the elve emission layers is also increased. This result reveals the leading role of atomic oxygen in the collocation. Furthermore, the altitude variation of elves is compared with the VLF reflection height observed by DEMETER. The latter height not only shows the SAO similarity to the elve/OH* heights, but also exhibits a difference between land and ocean. The possible relationship between VLF reflection height and elve/OH* is also discussed.

Description: Recent studies have reported that thermospheric effects due to solar wind driving can be observed poleward of auroral latitudes. In these papers, the measured neutral mass density perturbations appear as narrow, localized maxima in the cusp and polar cap. They conclude that Joule heating below the spacecraft is the cause of the mass density increases which are sometimes associated with local field-aligned current structures, but not always. In this paper we investigate neutral mass densities measured by accelerometers on the CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) spacecraft from launch until years 2010 (CHAMP) and 2012 (GRACE), approximately 10 years of observations from each satellite. We extract local maxima in neutral mass densities over the background using a smoothing window with size of one quarter of the orbit. The maxima have been analyzed for each year, and also for the duration of each set of satellite observations. We show where they occur, under what solar wind conditions, and their relation to magnetic activity. The region with the highest frequency of occurrence coincides approximately with the cusp and mantle, with little direct evidence of an auroral zone source. Our conclusions agree with the “hot polar cap” observations which have been reported and studied in the past.