Lyman-alpha measurements of neutral hydrogen in the outer geocorona and in interplanetary space

Thomas, Gary E.; Bohlin, R.

doi:10.1029/ja077i016p02752

Cited by 52 publications

(40 citation statements)

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“…This density enhancement is visible in the model fit for solar minimum as well as in the fit for solar maximum. An adequate phenomenon was first mentioned by Thomas and Bohlin (1972) and has since then been referred to as the "H geotail". Our results suggest a density maximum at low distances (3 R e ) that appears post-midnight and is shifted towards the dawn direction (see Fig.…”

Section: Enhanced H Densities At the Earth's Nightside (H Geotail)mentioning

confidence: 99%

“…In the past, observations have been carried out with high-altitude rockets or satellites (e.g., Kupperian et al, 1959;Johnson, 1961;Rairden et al, 1986;Østgaard et al, 2003) and even by Apollo 16 from the Moon (Carruthers et al, 1976). Theoretical studies have been presented (e.g., Chamberlain, 1963;Thomas and Bohlin, 1972;Fahr and Shizgal, 1983;Bishop, 1991;Hodges Jr., 1994).…”

Section: Introductionmentioning

confidence: 99%

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Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission

2015

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Abstract. Circumterrestrial Lyman-α column brightness observations above 3 Earth radii (R e ) have been used to derive separate 3-D neutral hydrogen density models of the Earth's exosphere for solar minimum (2008, 2010) and near-solarmaximum (2012) conditions. The data used were measured by Lyman-α detectors (LAD1/2) onboard each of the TWINS satellites from very different orbital positions with respect to the exosphere. Exospheric H atoms resonantly scatter the near-line-center solar Lyman-α flux at 121.6 nm. Assuming optically thin conditions above 3 R e along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H-column density. We found significant differences in the density distribution of the terrestrial exosphere under different solar conditions. Under solar maximum conditions we found higher H densities and a larger spatial extension compared to solar minimum. After a continuous, 2-month decrease in (27 day averaged) solar activity, significantly lower densities were found. Differences in shape and orientation of the exosphere under different solar conditions exist. Above 3 R e , independent of solar activity, increased H densities appear on the Earth's nightside shifted towards dawn. With increasing distance (as measured at 8 R e ) this feature is shifted westward/duskward by between −4 and −5 • with respect to midnight. Thus, at larger geocentric distance the exosphere seems to be aligned with the aberrated Earth-solar-wind line, defined by the solar wind velocity and the orbital velocity of the Earth. The results presented in this paper are valid for geocentric distances between 3 and 8 R e .

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Section: Enhanced H Densities At the Earth's Nightside (H Geotail)mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission

2015

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“…In the past, observations have been carried out with high-altitude rockets or satellites (e.g., Kupperian et al, 1959;Johnson, 1961;Rairden et al, 1986;Østgaard et al, 2003) and even by Apollo 16 from the Moon (Carruthers et al, 1976). Numerous theoretical studies on this subject have been published (e.g., Chamberlain, 1963; J. H. Zoennchen et al: The response of the H geocorona between 3 and 8 R e to geomagnetic disturbances Thomas and Bohlin, 1972;Fahr and Shizgal, 1983;Bishop, 1991;Hodges Jr., 1994).…”

Section: Introductionmentioning

confidence: 99%

The response of the H geocorona between 3 and 8 Re to geomagnetic disturbances studied using TWINS stereo Lyman-α data

Zoennchen¹,

Nass²,

Fahr³

et al. 2017

Ann. Geophys.

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Abstract. Circumterrestrial Lyman-α column brightness observations from 3-8 Earth radii (R e ) have been used to study temporal density variations in the exospheric neutral hydrogen as response to geomagnetic disturbances of different strength, i.e., Dst peak values between −26 and −147 nT. The data used were measured by the two Lyman-α detectors (LAD1/2) onboard both TWINS satellites between the solar minimum of 2008 and near the solar maximum of 2013. The solar Lyman-α flux at 121.6 nm is resonantly scattered near line center by exospheric H atoms and measured by the TWINS LADs. Along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H column density, assuming optically thin conditions above 3 R e . In the case of the eight analyzed geomagnetic storms we found a significant increase in the exospheric Lyman-α flux between 9 and 23 % (equal to the same increase in H column density n H ) compared to the undisturbed case short before the storm event. Even weak geomagnetic storms (e.g., Dst peak values ≥ −41 nT) under solar minimum conditions show increases up to 23 % of the exospheric H densities. The strong H density increase in the observed outer exosphere is also a sign of an enhanced H escape flux during storms. For the majority of the storms we found an average time shift of about 11 h between the time when the first significant dynamic solar wind pressure peak (p SW ) hits the Earth and the time when the exospheric Lyman-α flux variation reaches its maximum. The results show that the (relative) exospheric density reaction of n H have a tendency to decrease with increasing peak values of Dst index or the Kp index daily sum. Nevertheless, a simple linear correlation between n H and these two geomagnetic indices does not seem to exist. In contrast, when recovering from the peak back to the undisturbed case, the Kp index daily sum and the n H essentially show the same temporal recovery.Keywords. Atmospheric composition and structure (airglow and aurora; pressure density and temperature) -meteorology and atmospheric dynamics (thermospheric dynamics)

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“…Johnson, 1961;Chamberlain, 1963;Thomas and Bohlin, 1972;Fahr and Shizgal, 1983;Rairden et al, 1986;Bishop, 1991;Hodges, 1994;Østgaard et al, 2003).…”

Section: Introductionmentioning

confidence: 99%

Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum

2013

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The Lyman-α Detectors (LAD) on board the two TWINS 1/2-satellites allow for the simultaneous stereo imaging of the resonant emission glow of the H-geocorona from very different orbital positions. Terrestrial exospheric atomic hydrogen (H) resonantly scatters solar Lyman-α (121.567 nm) radiation. During the past solar minimum, relevant solar parameters that influence these emissions were quite stable. Here, we use simultaneous LAD1/2-observations from TWINS1 and TWINS2 between June 2008 and June 2010 to study seasonal variations in the H-geocorona. Data are combined to produce two datasets containing (summer) solstice and (combined spring and fall) equinox emissions. In the range from 3 to 10 Earth radii (R_E), a three-dimensional (3-D) mathematical model is used that allows for density asymmetries in longitude and latitude. At lower geocentric distances (< 3 R_E), a best fitting r-dependent (Chamberlain, 1963)-like model is adapted to enable extrapolation of our information to lower heights. We find that dawn and dusk H-geocoronal densities differ by up to a factor of 1.3 with higher densities on the dawn side. Also, noon densities are greater by up to a factor of 2 compared to the dawn and dusk densities. The density profiles are aligned well with the Earth–Sun line and there are clear density depletions over both poles that show additional seasonal effects. These solstice and equinox empirical fits can be used to determine H-geocoronal densities for any day of the year for solar minimum conditions

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Lyman-alpha measurements of neutral hydrogen in the outer geocorona and in interplanetary space

Cited by 52 publications

References 10 publications

Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission

Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission

The response of the H geocorona between 3 and 8 <i>R</i><sub>e</sub> to geomagnetic disturbances studied using TWINS stereo Lyman-<i>α</i> data

Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum

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Lyman-alpha measurements of neutral hydrogen in the outer geocorona and in interplanetary space

Cited by 52 publications

References 10 publications

Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission

Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission

The response of the H geocorona between 3 and 8 &lt;i&gt;R&lt;/i&gt;&lt;sub&gt;e&lt;/sub&gt; to geomagnetic disturbances studied using TWINS stereo Lyman-&lt;i&gt;α&lt;/i&gt; data

Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum

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The response of the H geocorona between 3 and 8 <i>R</i><sub>e</sub> to geomagnetic disturbances studied using TWINS stereo Lyman-<i>α</i> data