Earth Camera Research Articles

Earth Camera Network with DAS systems for non-invasive wildlife migration datasets and inter species communication.

Earth Camera Network with DAS systems for non-invasive wildlife migration datasets and inter species communication.

Development and In-Orbit Operation of Deep Learning Attitude Sensor

This study presents in-orbit demonstrations of a deep learning attitude sensor (DLAS) aboard the Japan Aerospace Exploration Agency (JAXA) small-satellite, Rapid Innovative Payload Demonstration Satellite 1. The DLAS is a spaceborne component that consists of star trackers, Earth cameras, and onboard computers. We propose a novel attitude determination method that uses a small camera and image recognition algorithm on board the DLAS to achieve three-axis attitude determination with minimal weight and volume. The DLAS mission further includes a demonstration of a low-cost high-performance star tracker that integrates consumer products. The development of the DLAS began in 2016 when it was adopted as the theme of the JAXA Innovative Satellite Technology Demonstration program. It was launched into a sun-synchronous orbit at an altitude of 500 km in January 2019 for approximately one year. This paper describes the results obtained from the DLAS operation and its future applications. The DLAS demonstrated an Earth camera image identification accuracy of more than 70%. Moreover, the obtained attitude determination accuracy was approximately 2.8 deg for the Earth camera and approximately 14 arcs for the star trackers. The technologies developed in the DLAS have been implemented in a subsequent microsatellite (HIBARI) of the Tokyo Institute of Technology.

The large‐scale current system during auroral substorms

AbstractWe present an empirical model of the equivalent current system in the ionosphere during the peak of a classical bulge‐type auroral substorm. This model is derived from measurements made by ~110 ground magnetometer stations during 116 substorms. The data are temporally and spatially organized using global auroral images obtained by the Polar Visible Imaging System Earth Camera. The empirical equivalent current system displays three key features: a poleward shift of the westward electrojet connecting the postmidnight and premidnight components; a polar cap swirl; and significantly different magnitudes of the postmidnight and premidnight westward electrojets. This leads us to propose a two‐wedge current system linking the ionosphere to the magnetosphere. The bulge current wedge is located in the premidnight region just equatorward of the open‐closed field line boundary while another three‐dimensional current system is located in the postmidnight region well within the auroral oval. We use Biot and Savart calculations and Tsyganenko mapping and show that this new model is a likely solution for the large‐scale current system.

Are there optical differences between storm-time substorms and isolated substorms?

Abstract. We have performed an extensive analysis of auroral optical events (substorms) that occurred during the development of the main phase of magnetic storms. Using images from the Earth Camera on the Polar spacecraft (Frank et al., 1995), we compared the optical emission features of substorms occurring during 16 expansion phases of magnetic storms with the features of isolated substorms occurring during non-storm times. The comparison used two techniques, visual inspection and statistical comparisons. The comparisons were based on the common characteristics seen in isolated substorms that were initially identified by Akasofu (1964) and quantified by Gjerloev et al. (2008). We find that when auroral activity does occur during main phase development the characteristics of the aurora are very dissimilar to those of the classical isolated substorm. The primary differences include the lack of a surge/bulge, lack of bifurcation of the aurora, much shorter expansion phases, and greater intensities. Since a surge/bulge and bifurcation of the aurora are characteristics of the existence of a substorm current wedge, a key component of the magnetosphere-ionosphere current system during substorms, the lack of this component would indicate that the classical substorm model does not apply to the storm time magnetosphere-ionosphere current system. Rather several of the analyses suggest that the storm-time substorms are associated more closely with the auroral oval, at least spatially, and, therefore, probably with the plasma sheet dynamics during the main phase development. These results then must call into question the widely held assumption that there is no intrinsic difference between storm-time substorms and classical isolated substorms.

Typical auroral substorm: A bifurcated oval

Utilizing global auroral images obtained by Polar Visible Imaging System Earth Camera, we have analyzed the UV emissions from 116 auroral substorms. The events selected were fairly isolated and had to expand from a localized onset. It was found that essentially all auroral events fulfilling these simple criteria expand into the Akasofu (1964) bulge‐type aurora. Averaged auroral emission patterns were compiled for 11 time steps of the substorm covering 20 min prior to the onset until well into the recovery phase. This compilation required a three‐step normalization technique, one temporal based on the expansion time and two spatial, magnetic local time and latitude. These average patterns were then fit to either a single or double Gaussian distribution in latitude for each of 24 magnetic local times. On the basis of this analysis we made the following conclusions. The normalization technique is highly efficient in maintaining the key features in the individual auroral emission patterns, even though the individual events varied significantly in intensity, size, position, and lifetime. Thus our normalization results quantitatively validate the Akasofu (1964) assumption that key auroral features exist in the bulge‐type auroral substorm. After the substorm onset the auroral oval becomes clearly bifurcated consisting of two components: the oval aurora in the latitude range of the preonset oval and expanding primarily eastward postmidnight, and the bulge aurora, which emerges out of the oval, expanding poleward and both east and west in MLT. The oval aurora decay faster than the bulge emissions indicating that the decays have separate time constants. Owing to the pronounced difference in the spatiotemporal behavior of the two auroral components we suggest that their sources are quasi‐independent.

Auroral conjugacy studies based on global imaging

Simultaneous global imaging in the ultraviolet wavelengths by IMAGE and Polar, enable us to examine auroral features in the conjugate hemispheres. With an imaging cadence of 2 and 1 min for IMAGE-FUV and Polar VIS Earth camera, respectively, we have examined dynamic features such as substorm onsets and cusp precipitation as well as slowly varying phenomena such as theta aurora. In this paper, we review the main findings from these studies and present new evidence of the IMF clock angle control of the asymmetric substorm onset locations. Simultaneous images from the opposite hemispheres show asymmetric cusp auroras and their locations are controlled by IMF B y and dipole tilt angle. Our imaging results demonstrate that theta aurora can be a non-conjugate phenomenon. We suggest that IMF B x controls in which hemisphere the theta aurora occurs. For substorm onset locations, we have found that there exists a systematic displacement in one hemisphere compared to the other. The relative displacement of onset locations in the conjugate hemispheres is found to be controlled primarily by the IMF clock angle. Compared with some of the existing magnetic field models, the observed asymmetries are an order of magnitude larger than the model predictions. Statistical distribution of substorm onset locations in the southern and northern hemispheres for different clock angles enables us to validate the IMF clock angle control. Based on ∼ 3000 substorm onsets in the northern hemisphere and ∼ 1000 in the southern hemisphere observed by IMAGE we find a remarkable support for our previous findings.

Observations and model predictions of substorm auroral asymmetries in the conjugate hemispheres

Based on imaging data from the Polar VIS Earth camera and the IMAGE‐FUV instruments we have documented how the IMF orientation and the dipole tilt angle act as the main and the secondary controlling factors of the relative displacement of the aurora in the conjugate hemispheres. Comparing our results with the asymmetries predicted by recent empirical magnetospheric models, we show that the assumed partial penetration of the IMF into the magnetosphere is clearly supported by observations, but the modeled conjugate shifts were found to be an order of magnitude smaller than the observed ones.

Interplanetary magnetic field control of the location of substorm onset and auroral features in the conjugate hemispheres

During 2001 and 2002, when the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite had its apogee in the Northern Hemisphere and the Polar spacecraft, owing to the apsidal precession of its orbit, reached higher altitudes in the Southern Hemisphere, the two spacecraft offered a unique opportunity to study the aurora in the conjugate hemispheres simultaneously. Owing to the large fields of view of the Polar Visible Imaging System (VIS) Earth camera and the IMAGE‐FUV instruments, substorms and auroral features were imaged on a global scale in both hemispheres. We have identified five substorm onsets and several auroral features that can be unambiguously identified and compared in the two hemispheres. When mapped onto apex coordinates in the two hemispheres, we find that substorm onset locations and auroral features are usually not symmetric. The longitudinal displacement in one hemisphere compared with the other can be as much as 1.5 hours of local time (∼1500 km). For southward interplanetary magnetic field (IMF) the hemispherical asymmetry (ΔMLT) is strongly correlated with the IMF clock angle (θC) and a linear fit, ΔMLT = −0.017θC + 3.44, gives a correlation coefficient of 0.83 with a mean deviation of 0.4ΔMLT. These findings are interpreted as the magnetic tensions force acting on open magnetic field lines before reconnecting in the magnetotail. This can also be thought of as the IMF penetrating the magnetosphere.

Temporal and spatial aspects of the cusp inferred from local and global ground‐ and space‐based observations in a case study

Global and local observations, from both ground and space facilities, are brought to bear to understand the origin of a stepped cusp ion dispersion observed 3 hours prenoon under an interplanetary magnetic field characterized by strongly positive By, conditions under which the cusp is believed to be shifted postnoon. Our main aim is to distinguish the temporal from the spatial cusp features. The local observations from Svalbard are from approximately the 0900–1200 MLT sector, while the in situ observations are from Polar at 0900 MLT. The global auroral observations were acquired by the Earth camera on Polar. The aurora from the ground perspective appeared as a periodic sequence of brightening events located on either side of a “midday gap aurora” centered at about 1030 MLT. At noon the aurora appeared as a semiperiodic sequence of brightening events. These were concurrent with enhancements of the overhead ionospheric Hall current. Polar observed two steps of a staircase ion energy‐latitude dispersion. These two steps were, in turn, concurrent with two consecutive auroral brightening events. We argue from this complement of observations that the stepped cusp was temporal in nature and resulted from a time‐varying reconnection process. Global UV imagery was employed to complement the ground observations for the purpose of separating the temporal from the spatial aspects. The imager captured, namely, two bands of strong UV emission in the prenoon and postnoon sectors, between which there appeared a persistent, weak UV aurora. We interpret this to be an underlying spatial substratum. Patchy and bursty UV enhancements coincided with the transients seen in the local ground and space measurements, corroborating further our distinction between space and time in the cusp. Comparing these inferences with recent work on the response of the dayside aurora to reconnection at the magnetopause, we find a close correspondence. In addition, we document the field‐aligned currents associated with the stepped cusp, which, in turn, compare favorably with recent theoretical ideas. This work is a contribution to the current debate on the nature of the stepped cusp and suggests that multiple data sets are essential to study this problem.

Observations of non‐conjugate theta aurora

Based on simultaneous imaging data from IMAGE FUV and Polar VIS Earth camera and DMSP particle measurements we report two events where a theta aurora was observed in one hemisphere but not in the other. IMF control and hemispherical conductivity differences are suggested as possible explanations for the occurrence of non‐conjugate theta aurora.

Simultaneous images of the northern and southern auroras from the Polar spacecraft: An auroral substorm

The first simultaneous images of the auroras in the Northern and Southern Hemispheres with a single camera have been gained with the Earth Camera onboard the Polar spacecraft with sufficient angular resolution to provide maps of the intensities. An auroral substorm was fortuitously observed on 1 November 2001 when the spacecraft was favorably positioned to observe both auroras. The auroral intensities during the onset and the early expansive phase were mapped into corrected geomagnetic coordinates. The onset brightening at far‐ultraviolet wavelengths was first detected in the southern auroras and subsequently seen in the northern auroras about 1 min later on L‐shells of about 5 RE. During the early phases of the auroral expansive phase, the southern aurora continued to be brighter. The unequal auroral intensities in the two hemispheres suggest that there is strong coupling of the ionosphere and magnetosphere. The mapping of the auroras revealed that there is a local time difference between the onset positions of about 40 min, with the southern auroras positioned to the east relative to those in the north. This displacement was also similarly detected in the quiet auroral regions, which were not participants in the auroral substorm. Such displacements may be useful in developing improvements in the accuracy of global magnetic models that are important for determination of the position of the in situ particle and field measurements in the magnetosphere. The greater brightness of the northern quiet time auroras by tens of percent as first reported by aircraft observations at visible wavelengths in both hemispheres has been verified by the more global view from the Polar spacecraft.

Observations of plasma injection into the ring current during substorm expansive phase

The injection of plasmas during the expansive phase of four substorms was investigated with global auroral images and simultaneous measurements of plasmas and magnetic fields in the plasma sheet near the magnetic equator. The high‐resolution images were gained at far‐ultraviolet wavelengths with the Earth Camera on board the Polar spacecraft and the observations of plasmas and magnetic fields were acquired with the Geotail spacecraft at radial distances in the range of 11 to 15 RE. For each of the substorms an ion flow burst was detected with maximum speeds in the range of 200 to 400 km/s and durations of several minutes. The ion bulk flow was directed earthward. For the first time auroral images recorded the position of the flow bursts as the leading edge of the westward or eastward traveling surge during the expansive phase of the substorm. The time intervals from the substorm onset as recorded by the camera until the detection of the flow pulse at the Geotail spacecraft ranged from 3 to 11 min. With the exception of one event which was located near the duskside magnetopause, the ion velocity distributions were characterized by the same densities and temperatures during and after the flow pulse for each of the individual events. For three of the events the electron temperature increased by a factor of about 2 after passage of the flow pulse. For the fourth event the spacecraft was not in the plasma sheet prior to the flow pulse, and hence this temperature increase could not be determined. Substantial fluctuations of the magnetic fields accompany the flow pulses. The electron and ion spectra were smoothly varying during the flow pulses without evidence of the highly structured spectral features due to combined gradient and electric field drifts which are well documented for particle injections near the 6.6‐RE synchronous orbit. The measurements at the Geotail spacecraft position support the existence of the previously proposed “injection front” for plasmas which is due to transient earthward convection during the expansive phase of substorms. This convection occurs during the diversion of the equatorial current sheet into the ionosphere at locations inside the Geotail position.

Interplanetary magnetic field By dependence of the relative position of the dayside ultraviolet auroral oval and the HF radar cusp

Comparative analyses of CUTLASS Super Dual Auroral Radar Network HF radar data and images of the dayside ultraviolet aurora from the Polar far ultraviolet imager and the Visible Imaging System Earth Camera have revealed that the GSM y component of the interplanetary magnetic field (By) affects the relative positions of ultraviolet auroral emissions and the HF radar signature of the cusp in the postnoon sector during episodes of flux transfer at the magnetopause. When By is positive, the region of strong auroral emissions is found to be at the same latitude as the radar footprint of the cusp. When By is negative, the radar cusp is found to be poleward of the strong auroral emissions. These results are consistent with models of the ionospheric convection and field‐aligned current response to reconnection at the magnetopause under positive and negative By conditions. We extend the models to predict the auroral and convection responses in the morning sector.

Observations of plasma sheet dynamics earthward of the onset region with the Geotail spacecraft

During the onset of an auroral substorm at about 0406 UT on August 14, 1996, the position of the Geotail spacecraft was located very near the corresponding region of instability in the equatorial plane as determined with a global model of the geomagnetic fields which provided the mapping from the auroral ionosphere. The Earth Camera on board the Polar spacecraft was used to acquire the global auroral images. The magnetic fields in the ring current were sufficiently stretched by the equatorial current sheet such that the radial position of the Geotail spacecraft at 10.2 RE was earthward of the onset region. Earthward flows at speeds in the range of 200 km/s for a period of several minutes were coincident with the initial brightening of a localized region of the previously existing auroral oval. This auroral luminosity subsequently declined within several minutes without a well‐developed expansive phase; that is, a “pseudo‐onset” occurred. Partial dipolarization of the magnetic fields occurred with this onset as well as large‐scale magnetic fluctuations which continued through the brightening of another spot on the auroral oval at 0413 UT. These fluctuations ceased with the end of a well‐developed expansive phase at about 0424 UT, when the auroral luminosities entered their recovery phase. At the beginning of the short expansive phase at 0420 UT the magnetometer on GOES 8 at a radial distance of 6.6 RE recorded a major dipolarization of the magnetic fields as an earthward moving injection front of hot plasma passed across the Geotail position. The present observations of the onset phenomena earthward of the instability region are combined with previously reported measurements of plasmas and magnetic fields at distances tailward of the instability in the ring current in order to begin sketches of the relationship among several dynamical features of the onset region.

The dayside ultraviolet aurora and convection responses to a southward turning of the interplanetary magnetic field

Abstract. We examine the large-scale ultraviolet aurora and convection responses to a series of flux transfer events that immediately followed a sharp and isolated southward turning of the IMF. During the interval of interest, SuperDARN was monitoring the plasma convection in the dayside northern ionosphere, while the VIS Earth Camera and the Far Ul-traviolet Imager (UVI) were monitoring the northern hemisphere’s ultraviolet aurora. Reconnection signatures were seen in the SuperDARN HF radar data in the postnoon sector following a sharp southward turning of the IMF. The presence of flux transfer events is supported by measurements of a classic dispersed ion signature in the low-altitude cusp from the DMSP spacecraft. Subsequent to the onset of reconnection, the postnoon convection and ultraviolet aurora expanded in concert, reaching 18 MLT in half an hour. The auroral oval was found to move equatorward at the convection speed in the 16–18 MLT sector, implying that it was related directly to an adiaroic magnetospheric boundary. In the present study, we have estimated the field-aligned current response to magnetic reconnection in terms of the vorticity of the ionospheric plasma convection velocity. The convection velocities were obtained using two methods: (a) direct reconstruction of the full vector velocities from bistatic measurements of the convection by the SuperDARN HF radars in a relatively small region of the auroral zone, and (b) from global-scale spherical harmonic fits to the SuperDARN velocities deduced from the map potential model. Regions of high vorticity, which were predicted to be an estimate of a component of the total field-aligned current, agree extremely well with the images of the dayside UV aurora, indicating that, in this case, the plasma vorticity is an excellent estimator of the morphology of dayside field-aligned currents (FACs). The morphology of the aurora and ionospheric electric field in the postnoon sector supports the existence of a dayside current wedge induced in response to dayside reconnection.Key words. Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions; solar wind magne-tosphere interactions)

Two encounters of the substorm onset region with the Geotail spacecraft

The Earth Camera on board the Polar spacecraft provided high‐resolution images for 112 auroral substorms when the Geotail spacecraft was in the nighttime sector of Earth's magnetotail during the period May 1 through September 30, 1996. The purpose of the study was to investigate the dynamical behavior of the plasmas and magnetic fields at the Geotail spacecraft when it was positioned within the central plasma sheet prior to and during the onset of a substorm. In addition, the position of the Geotail spacecraft was required to be within 20 min in local time and several degrees in magnetic latitude of the equatorial position of the magnetic field lines threading the auroral onset region as viewed with the Polar camera and as mapped with a global magnetic field model. For this study these onset mapping locations were required to be earthward of the Geotail spacecraft. Only two substorms, on July 2 and September 4, satisfied these restraints which were imposed in order to observe the dynamics of the central plasma sheet. The radial distances for the Geotail locations for these two substorms were 11.7 and 15.0 RE, respectively. Before the onsets there was a sustained period of earthward flow of plasma into the extraterrestrial ring current at average speeds of ∼50 km s−1. Then, for a period of several minutes before the auroral onset, this plasma flow became stagnant and was accompanied, in part, by fluctuations in components of the magnetic field. At substorm onset a tailward high‐speed flow of plasmas at 200–300 km S−1 began which subsequently subsided within several minutes. The magnetic fields in this tailward directed pulse of plasma flows exhibited a substantial southward component which is accounted for by magnetic merging of field lines at locations inside of the Geotail spacecraft orbit and thus in the ring current or earthward edge of the plasma sheet. Overall, these fortuitous observations support the merging of magnetic field lines at substorm onset in the plasmas with high‐energy densities in the ring current but within a very restricted range of local time, several tens of minutes or less.

The UV aurora and ionospheric flows during flux transfer events

Abstract. Far Ultra Violet (FUV) signatures in the polar ionosphere during a period of magnetopause reconnection are compared with ionospheric flows measured in the cusp ‘throat’ and dusk cell by the CUTLASS Hankasalmi HF radar. Regions of peak FUV emission in the 130.4 nm and 135.6 nm range, observed by the Polar spacecraft’s VIS Earth Camera, consistently lie at the turning point of the flows from the dusk cell, poleward into the throat, and at the equatorward edge of the region of high and varied radar spectral-width associated with the cusp. The Equator-S spacecraft was near the magnetopause at the time of the ionospheric observations and geomagnetically conjugate with the region of ionosphere observed by the radar. Flux transfer events (FTEs), suggestive of bursty reconnection between the IMF and geomagnetic fields, were observed by Equator-S prior to and during the periods of high FUV emission. Enhanced poleward ionospheric flow velocities in the polar cusp region, previously shown to be associated with bursty reconnection, consistently lie poleward of the enhanced FUV optical feature. The enhanced optical feature is consistent with the expected position of the largest upward region 1 field-aligned current, associated with electron precipitation, on the dusk edge of the merging gap. The optical feature moves duskward and equatorward during the course of the reconnection sequence, consistent with expansion of the merging line and the polar cap with newly added open magnetic flux by the FTEs. The DMSP F14 spacecraft passed through the enhanced FUV region and measured strong, structured electron precipitation far greater than in the adjacent regions.Key words. Magnetospheric physics (current systems; magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions)

Atmospheric holes: Instrumental and geophysical effects

Global images of far‐ultraviolet dayglow with the Earth Camera on board the Polar spacecraft were used for a comprehensive study of transient, localized decreases of intensities, also known as atmospheric holes. An automated determination for detection of an atmospheric hole was developed and employed in this survey of the characteristics of this phenomenon, which avoided the possibility of aliasing by visual inspection of the images. An extensive investigation of possible instrumental effects included (1) evaluation of random rates for a period in mid‐January for which there were no atmospheric holes, thus providing an excellent inflight calibration of the instrument's noise, (2) the effects in the images due to energetic electrons in the outer radiation zone, (3) the nonuniform sensitivities for the pixels of the sensor, or “hot spots,” and (4) the contributions of longer wavelength radiation from the atmosphere to the camera's responses. For the straightforward selection criteria for the identification of atmospheric holes the instrumental effects were not major contributors to the occurrence frequencies of this phenomenon. The subsequent search for geophysical effects revealed (1) a strong altitude dependence of the frequency of atmospheric holes, (2) a substantial local‐time variation of rates which favored locations in the local‐morning sectors of the atmosphere relative to those in the evening, (3) increasing sizes of the atmospheric holes as the spacecraft altitude decreases, and (4) large seasonal variations in the hole rates during the period November 1997 through late January 1998 which were remarkably similar to those observed during the same months but 16 years earlier with Dynamics Explorer 1.

Revisiting the Rain of Cometary Snowballs

It's an intriguing idea: A steady rain of kiloton minicomets of loosely packed ice, pelting the top of the atmosphere at a rate of about a dozen per minute could, over geological time, account for most of the water in the oceans, and who knows what else. That's the picture suggested last year by Louis Frank and John Sigwarth at the University of Iowa, based on their observations with the Visual Imaging System (VIS), the Earth camera built by Frank and carried aboard NASA's Polar orbiter. (See PHYSICS TODAY, July 1997, page 18.)

Small‐comet “atmospheric holes” are instrument noise

Frank and Sigwarth claim that dark pixel clusters observed by the VIS Earth Camera are atmospheric holes created by small comets. We have examined their catalog, which contains about 700,000 of these “atmospheric holes”, for consistency with this small‐comet hypothesis and with instrument noise. A geometrical requirement of the small‐comet hypothesis is that the number of pixels in a typical cluster must vary by a factor > 100 with spacecraft altitude because of the inverse‐square law of the apparent cluster area versus distance. We find no systematic variation of duster size with spacecraft altitude. The Iowa catalog data are consistent with instrument noise because neither the size distribution nor the event rate of dark pixel clusters depend on altitude. At altitudes outside of the radiation belts during the one day of available raw data, more than 75% of the dark pixel dusters result from the process that Frank and Sigwarth employ to remove bright pixels caused by energetic particles. This data processing also causes additional meaningless dark pixel clusters to occur in the dark sky or over the dark Earth.