Guang-Ming Chen

A comparison of the effects of CIR- and CME-induced geomagnetic activity on thermospheric densities and spacecraft orbits: Case studies

Enhanced energy input from the magnetosphere to the upper atmosphere during geomagnetic storms has a profound effect on thermospheric density and consequently near-Earth satellite orbit decay. These geomagnetic storms are caused by two different processes. The first is coronal mass ejections (CMEs) and the second is corotating interaction regions (CIRs). CME-driven storms are characterized by large maximum energy input but relatively short duration, whereas CIR-driven storms have relatively small maximum energy input but are of a considerably longer duration. In this paper we carried out a statistical study to assess the relative importance of each kind of storm to satellite orbital decay. The results demonstrate that CIR storms have a slightly larger effect on total orbital decay than CME storms do in a statistical sense. During the declining phase and the minimum years of a solar cycle, CIR storms occur frequently and quasiperiodically. These storms have a large effect on thermospheric densities and satellite orbits because of their relatively long duration. Thus, it is important to fully understand their behavior and impact.

The effect of periodic variations of thermospheric density on CHAMP and GRACE orbits

[1] In this paper thermosphere densities observed by the CHAMP and GRACE satellites and their orbital parameters are used to investigate the effect of periodic oscillations in thermospheric densities (7–27 days) caused by solar rotation and periodic magnetic activity on satellite orbits during 2003–2005. Two new results are obtained in this study. First, the response of the mean radius of the satellite orbit per revolution (MRPR) to the oscillations in the mean atmospheric density per revolution (MDPR) increased linearly with oscillation periods. Therefore, MRPR had a strong oscillation near the 27 day period. However, it had no obvious 7, 9, and 13.5 day oscillations, although there were strong oscillations at the same periods in MDPR. Second, there was a phase difference of � =2 between the oscillations of MRPR and MDPR. The phases of the oscillations in MRPR led the phases of the variations in MDPR. The correlation coefficient between the 27 day oscillations in MRPR and those in MDPR was 0.83 with a phase difference of −6.8 days for CHAMP; the correlation for GRACE was 0.67 with a phase difference of −6.4 days. The amplitudes of the oscillations in MRPR of CHAMP were larger than those of GRACE because GRACE had a higher orbit than CHAMP. These features are in good agreement with our theoretical analysis.

The responses of the nightglow emissions observed by the TIMED/SABER satellite to solar radiation

The responses of four nightglow emissions, NO emission at 5.3 mu m, O-2 infrared atmospheric band at 1.27 mu m, and OH emissions at 2.0 mu m and 1.6 mu m (referred to as OH2 and OH1 in this study), to solar radiation are studied and compared based on the data observed by the Sounding of the Atmosphere using Broadband Emission Radiometry instrument over 13years. The quantitative relationships between the nightglow emissions and solar radiation are obtained by a linear regression fit using the F-10.7 index. The intensities and the peak heights of the 13year average global mean NO, O-2, OH2, and OH1 nightglows are 270.042.8kR, 106.92.2kR, 133.21.6kR, 217.52.4kR, 123.6 +/- 0.2km, 89.8 +/- 0.05km, 88.1 +/- 0.02km, and 86.6 +/- 0.02km, respectively. Among the four nightglow emissions, the influence of solar radiation on the ones at lower heights is weaker than the ones higher above. The responses of the global mean NO, O-2, OH2, and OH1 nightglow intensities to solar radiation are 176.3 +/- 4.8%/100solar flux units (sfu), 22.2 +/- 1.4%/100sfu, 12.9 +/- 1.1%/100sfu, and 11.4 +/- 1.3%/100sfu, respectively. The intensities and peak emission rates of the four global mean nightglow emissions are highly correlated to solar radiation. The response of the height of the global mean O-2 nightglow peak emission rate to solar radiation is 0.51 +/- 0.08km/100sfu. The responses of NO, OH2, and OH1 nightglow peak heights to solar radiation are not obvious. In addition, the responses of nightglow emissions to solar radiation change with latitude.

Double-layer structure of OH dayglow in the mesosphere

Observations by the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite from January 2002 to June 2014 are used to study the vertical structure of OH dayglow. The results indicate for the first time that there is a double-layer structure in the distributions of 12year averaged OH airglow emission, [O-3], and [H] during the daytime. The upper layer of OH dayglow is located in the mesopause region (similar to 88km) at a similar altitude to that of the OH nightglow. The lower layer is situated in the range of 70-85km. Both the peak emission and height of the lower layer increase with local time. The distance between the two layers decreases with local time. At the equator, the lower layer forms at similar to 09:00 LT and lasts for about 8h; during this time the interlayer distance decreases from 13km to 5km. The double-layer structure is more obvious and longer-lived during the equinoxes and at lower latitudes. The double-layer structure of OH dayglow emission is a long-term stable structure and is mainly caused by photochemical processes involving [O-3]. It is also modulated by background atmospheric temperature and [H].

Impact of Solar Forcing on Thermospheric Densities and Spacecraft Orbits from CHAMP and GRACE

The thermosphere is the outer gaseous shell of a planet’s atmosphere that exchanges energy with the space plasma environment. The energy deposition of solar irradiation and magneto‐ spheric inputs into the upper atmosphere can change the thermospheric density significantly. From a practical standpoint, unanticipated changes in the density of the thermosphere cause satellites to deviate from their anticipated paths, or ephemerides. Many studies have been pursued to investigate the variations of thermospheric densities caused by solar forcing, which includes solar irradiation and magnetospheric energy deposition [1-12]. However, the quantitative examination of the impact of thermospheric density changes associated with solar forcing on satellite orbits is rare, given that the simultaneous measurements of thermospheric density and precise tracking data of satellite are sparse.

The responses of ionospheric topside diffusive fluxes to two geomagnetic storms in October 2002

O+ field-aligned ambipolar diffusive velocities V-d and fluxes Phi(d) in the topside ionosphere have been calculated from the observed profiles of electron density, ion, and electron temperatures during a 30 day incoherent scatter radar experiment conducted at Millstone Hill (288.5 degrees E, 42.6 degrees N) from 4 October to 4 November 2002. Two geomagnetic storms took place during this period. During the negative phases (depleted electron densities) of these two storms, the magnitudes of the daytime upward V-d and Phi(d) were less than their averaged quiet time values. Whereas at nighttime, the downward V-d and Phi(d) were sometimes larger than the averaged quiet time values. The variations in diffusive velocity and flux during the storm main and recovery phases were caused by changes in the ionospheric scale height or the shapes of ionospheric density profiles. The negative storm effect further reduced daytime diffusive flux. During these two storms, positive ionosphere phases (enhanced electron densities) were also observed. The diffusive velocity was much smaller during the period of positive storm effect, which led to a smaller diffusive flux than the quiet time one, although electron density was higher. It appears that storm time variations in diffusive velocity were more the results of storm time changes in the plasma vertical profile, rather than the cause of these plasma density changes.

Effects of solar proton events on dayglow observed by the TIMED/SABER satellite

The effect of solar proton events on the daytime O2 and OH airglows and ozone and atomic oxygen concentrations in the mesosphere are studied using data from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). Five events occurred in September 2005, December 2006, March 2012, May 2013, and June 2015 that satisfy two criteria: the maximum proton fluxes are larger than 1000 pfu and daytime data in the high latitude region are available from SABER. The event in December 2006 is studied in detail and the effects of all five events are compared in brief. The results indicate that all four parameters in the mesosphere decrease during the events. During the event in 2006, the maximum depletions of O2 and OH dayglow emission rates and ozone and atomic oxygen volume mixing ratios at 70 km are respectively 31.6%, 37.0%, 42.4%, and 38.9%. The effect of the solar proton event changes with latitude, longitude and altitude. The depletions due to the stronger events are larger on average than those due to the weaker events. The depletions of both dayglow emission rates are weaker than those of ozone and atomic oxygen. The responses of O2 and OH nightglow emissions around their peak altitudes to the SPEs are not as strong and regular as those for dayglow in the mesosphere.