Long-term ionospheric cooling: Dependency on local time, season, solar activity, and geomagnetic activity

Ionospheric ion temperature Ti is an excellent approximation to neutral temperature Tn in the thermosphere, especially for altitudes below 300 km. This analysis of long‒term Ti trends in the F region over different local times is based on a database of incoherent scatter radar (ISR) observations spanning more than three solar cycles during 1968–2006 at Millstone Hill and represents an extended effort to a prior study focusing on noon‒time only. This study provides important information for understanding the difference between the ISR and other results. A gross average of the Ti trend at heights of Ti ∼ Tn (200–350 km) is ∼ −4 K/decade, a cooling trend close to the Tn estimation based on the satellite neutral density data. However, there exists considerable variability in the cooling: it is strong during the day and very weak during the night with a large apparent warming at low altitudes (200–350 km); it is strong at solar minimum for both daytime and nighttime. The strongest cooling for altitudes below 375 km occurs around 90–120 solar flux units of the 10.7 cm solar flux, not at the lowest solar flux. There appears more cooling toward high magnetic activity, but this dependency is very weak. No consistent and substantial seasonal dependency across different heights was found. We speculate that a fraction of the observed cooling trend may be contributed by a gradual shifting away from the sub‒auroral region at Millstone Hill, as part of the secular change in the Earth's magnetic field. In this 39 year long series of data record, two anomalous Ti drops were noticed, and we speculate on their connection to volcano eruptions in 1982 and 1991.National Science Foundation (U.S.) (Award AGS-1042569

Day-to-day variability and solar preconditioning of thermospheric temperature over Millstone Hill

We use a continuous 30 day incoherent scatter radar experiment at Millstone Hill in October 2002 to examine day-to-day thermospheric variability in exospheric temperature T[subscript ex]. Solar flux and magnetic activity influences as the main driving factors for day-to-day variability are investigated quantitatively. Solar ultraviolet flux levels are based on the TIMED/SEE space weather product, allowing for analysis of ultraviolet flux-T[subscript ex] correlation. T[subscript ex] is most sensitive to solar EUV flux with approximately a 2 day delay at wavelengths of 27–34 nm (including 30.4 nm). In particularly, a 20–60 h time delay occurs in T[subscript ex] response to EUV flux at 27–34 nm band, with shorter delays in the morning and longer delays in the afternoon and at night. The 1 ∼ 2 day delayed T[subscript ex] response to solar ultraviolet flux and associated thermospheric solar preconditioning (“memory”) are most significant in the daily mean for the 27–34 nm band, in the diurnal and semidiurnal amplitudes for the soft X-ray flux at 0.1–7 nm, and in the diurnal amplitude for longer wavelengths. An empirical model driven only by EUV flux at 27–34 nm from 2 days in advance reproduces 90% of the observed variability in the Tex daily mean. With a 2 day time delay, solar X-ray flux at 0.1–7 nm is correlated positively with T[subscript ex] diurnal amplitude and negatively with T[subscript ex] semidiurnal amplitude. Finally, magnetic activity control, as represented by the Dst index, is weaker during the day and stronger at night and is important for the semidiurnal amplitude but not important for the daily mean.National Science Foundation (U.S.) (Award AGS-1042569

Negative Magnetoresistance in Dirac Semimetal Cd3As2

A large negative magnetoresistance is anticipated in topological semimetals in the parallel magnetic and electric field configuration as a consequence of the nontrivial topological properties. The negative magnetoresistance is believed to demonstrate the chiral anomaly, a long-sought high-energy physics effect, in solid-state systems. Recent experiments reveal that Cd3As2, a Dirac topological semimetal, has the record-high mobility and exhibits positive linear magnetoresistance in the orthogonal magnetic and electric field configuration. However, the negative magnetoresistance in the parallel magnetic and electric field configuration remains unveiled. Here, we report the observation of the negative magnetoresistance in Cd3As2 microribbons in the parallel magnetic and electric field configuration as large as 66% at 50 K and even visible at room temperatures. The observed negative magnetoresistance is sensitive to the angle between magnetic and electrical field, robust against temperature, and dependent on the carrier density. We have found that carrier densities of our Cd3As2 samples obey an Arrhenius's law, decreasing from 3.0x10^17 cm^-3 at 300 K to 2.2x10^16 cm^-3 below 50 K. The low carrier densities result in the large values of the negative magnetoresistance. We therefore attribute the observed negative magnetoresistance to the chiral anomaly. Furthermore, in the perpendicular magnetic and electric field configuration a positive non-saturating linear magnetoresistance up to 1670% at 14 T and 2 K is also observed. This work demonstrates potential applications of topological semimetals in magnetic devices

Statistical Analysis of the Main Ionospheric Trough Using Swarm in Situ Measurements

A statistical analysis of the topside main ionospheric trough is implemented by using the Swarm constellation in situ plasma density measurements from December 2013 to November 2019. The key features of the main trough, such as the occurrence rate, minimum position, width, and depth, are characterized and quantified. The distribution patterns of these parameters are investigated with respect to magnetic local time, season, longitude, solar activity, and geomagnetic activity levels, respectively. The main results are as follows: (1) The diurnal variation of the trough occurrence rate usually exhibits a primary peak in the early morning, a subsidiary peak in the late evening, and a slight reduction around midnight especially in the Northern Hemisphere. (2) The seasonal variation of the nighttime trough has maximum occurrence rates around equinoxes, higher than those in local winter. (3) The trough distribution has an evident hemispherical asymmetry. It is more pronounced in the Northern Hemisphere during the winter and equinoctial seasons, with its average nighttime occurrence rate being 20â 30% higher than that in the Southern Hemisphere. The trough minimum position and the trough width also exhibit more significant fluctuation in the Northern Hemisphere. (4) The longitudinal pattern of the trough shows clear eastâ west preferences, which has a higher occurrence rate in eastern (western) longitudes around the December (June) solstice. (5) Conditions for the trough occurrence are more favored in low solar activity and high geomagnetic activity periods.Key PointsThe occurrence rate of the main ionospheric trough at 450â 550 km exhibits a slight midnight reduction comparing with evening/morning peaksThe trough has a longitudinal preference with higher occurrence rate in the eastern (western) longitudes around the December (June) solsticeConditions for the trough occurrence are more favored in equinoxes than local winter and in Northern Hemisphere than Southern HemispherePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154408/1/jgra55592.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154408/2/jgra55592_am.pd

Ionospheric symmetry caused by geomagnetic declination over North America

We describe variations in total electron content (TEC) in the North American sector exhibiting pronounced longitudinal progression and symmetry with respect to zero magnetic declination. Patterns were uncovered by applying an empirical orthogonal function (EOF) decomposition procedure to a 12 year ground-based American longitude sector GPS TEC data set. The first EOF mode describes overall average TEC, while the strong influence of geomagnetic declination on the midlatitude ionosphere is found in the second EOF mode (or the second most significant component). We find a high degree of correlation between spatial variations in the second EOF mode and vertical drifts driven by thermospheric zonal winds, along with well-organized temporal variation. Results strongly suggest a causative mechanism involving varying declination with longitude along with varying zonal wind climatology with local time, season, and solar cycle. This study highlights the efficiency and key role played by the geomagnetic field effect in influencing mesoscale ionospheric structures over a broad midlatitude range.National Science Foundation (U.S.) (Grant ATM-0733510)National Science Foundation (U.S.) (Grant ATM-0856093)National Science Foundation (U.S.) (Grant AGS-1242204)China Scholarship CouncilHaystack Observator

Ionospheric longitudinal variations at midlatitudes: Incoherent scatter radar observation at Millstone Hill

Incoherent scatter radar (ISR) extra-wide coverage experiments during the period of 1978–2011 at Millstone Hill are used to investigate longitudinal differences in electron density. This work is motivated by a recent finding of the US east-west coast difference in TEC suggesting a combined effect of changing geomagnetic declination and zonal winds. The current study provides strong supporting evidence of the longitudinal change and the plausible mechanism by examining the climatology of electron density Ne on both east and west sides of the radar with a longitude separation of up to 40o for different heights within 300–450 km. Main findings include: 1) The east-west difference can be up to 60% and varies over the course of the day, being positive (East side Ne > West side Ne) in the late evening, and negative (West side Ne > East side Ne) in the pre-noon. 2) The east-west difference exists throughout the year. The positive (relative) difference is most pronounced in winter; the negative (relative) difference is most pronounced in early spring and later summer. 3) The east-west difference tends to enhance toward decreasing solar activity, however, with some seasonal dependence; the enhancements in the positive and negative differences do not take place simultaneously. 4) Both times of largest positive and largest negative east-west differences in Ne are earlier in summer and later in winter. The two times differ by 12–13 h, which remains constant throughout the year. 5) Variations at different heights from 300–450 km are similar. Zonal wind climatology above Millstone Hill is found to be perfectly consistent with what is expected based on the electron density difference between the east and west sides of the site. The magnetic declination-zonal wind mechanism is true for other longitude sectors as well, and may be used to understand longitudinal variations elsewhere. It may also be used to derive thermospheric zonal winds.National Natural Science Foundation (China) (Grant 40890164)National Science Foundation (U.S.) (Grants ATM-0733510 and ATM- 6920184

GNSS Observations of Ionospheric Variations During the 21 August 2017 Solar Eclipse

An edited version of this paper was published by AGU. Copyright 2017 American Geophysical Union. Coster, A.J., Goncharenko, L., Zhang S.-R., Erickson, P.J., Rideout, W. & Vierinen, J. (2017). GNSS Observations of Ionospheric Variations During the 21 2 August 2017 Solar Eclipse. Geophysical Research Letters, 44(24), 12041-12048. https://doi.org/10.1002/2017GL075774. To view the published open abstract, go to https://doi.org/10.1002/2017GL075774.On 21 August 2017, during daytime hours, a total solar eclipse with a narrow ∼160 km wide umbral shadow occurred across the continental United States. Totality was observed from the Oregon coast at ∼9:15 local standard time (LST) (17:20 UT) to the South Carolina coast at ∼13:27 LST (18:47 UT). A dense network of Global Navigation Satellite Systems (GNSS) receivers was utilized to produce total electron content (TEC) and differential TEC. These data were analyzed for the latitudinal and longitudinal response of the TEC and for the presence of traveling ionospheric disturbances (TIDs) during eclipse passage. A significant TEC depletion, in some cases greater than 60%, was observed associated with the eclipse shadow, exceeding initial model predictions of 35%. Evidence of enhanced large‐scale TID activity was detected over the United States prior to and following the large TEC depletion observed near the time of totality. Signatures of enhanced TEC structures were observed over the Rocky Mountain chain during the main period of TEC depletion

Ionospheric ion temperature climate and upper atmospheric long-term cooling

It is now recognized that Earth's upper atmosphere is experiencing a long-term cooling over the past several solar cycles. The potential impact of the cooling on societal activities is significant, but a fundamental scientific question exists regarding the drivers of the cooling. New observations and analyses provide crucial advances in our knowledge of these important processes. We investigate ionospheric ion temperature climatology and long-term trends using up-to-date large and consistent ground-based data sets as measured by multiple incoherent scatter radars (ISRs). The very comprehensive view provided by these unique observations of the upper atmospheric thermal status allows us to address drivers of strong cooling previously observed by ISRs. We use observations from two high-latitude sites at Sondrestrom (invariant latitude 73.2°N) from 1990 to 2015 and Chatanika/Poker Flat (invariant latitude 65.9°N) over the span of 1976–2015 (with a gap from 1983 to 2006). Results are compared to conditions at the midlatitude Millstone Hill site (invariant latitude 52.8°N) from 1968 to 2015. The aggregate radar observations have very comparable and consistent altitude dependence of long-term trends. In particular, the lower F region (<275 km) exhibits dayside cooling trends that are significantly higher (−3 to −1 K/yr at 250 km) than anticipated from model predictions given the anthropogenic increase of greenhouse gases. Above 275 km, cooling trends continue to increase in magnitude but values are strongly dependent on magnetic latitude, suggesting the presence of significant downward influences from nonneutral atmospheric processes.National Science Foundation (U.S.) (Awards AGS-1042569 and AGS-1343056

Coordinated Groundâ Based and Spaceâ Based Observations of Equatorial Plasma Bubbles

This paper presents coordinated and fortuitous groundâ based and spaceborne observations of equatorial plasma bubbles (EPBs) over the South American area on 24 October 2018, combining the following measurements: Globalâ scale Observations of Limb and Disk far ultraviolet emission images, Global Navigation Satellite System total electron content data, Swarm in situ plasma density observations, ionosonde virtual height and drift data, and cloud brightness temperature data. The new observations from the Globalâ scale Observations of Limb and Disk/ultraviolet imaging spectrograph taken at geostationary orbit provide a unique opportunity to image the evolution of plasma bubbles near the F peak height over a large geographic area from a fixed longitude location. The combined multiâ instrument measurements provide a more integrated and comprehensive way to study the morphological structure, development, and seeding mechanism of EPBs. The main results of this study are as follows: (1) The bubbles developed a westward tilted structure with 10â 15° inclination relative to the local geomagnetic field lines, with eastward drift velocity of 80â 120 m/s near the magnetic equator that gradually decreased with increasing altitude/latitude. (2) Waveâ like oscillations in the bottomside F layer and detrended total electron content were observed, which are probably due to upward propagating atmospheric gravity waves. The wavelength based on the mediumâ scale traveling ionospheric disturbance signature was consistent with the interbubble distance of â ¼500â 800 km. (3) The atmospheric gravity waves that originated from tropospheric convective zone are likely to play an important role in seeding the development of this equatorial EPBs event.Plain Language SummaryThis study presents multiâ instrument observations of equatorial plasma density depletions occurred on 24 October 2018 by using Globalâ scale Observations of Limb and Disk far ultraviolet images, Global Navigation Satellite System total electron content data, electron density measurements from Swarm satellite, ionosonde measurements, and cloud temperature data. This multiâ instrument study generated an integrated and detailed image revealing both largeâ scale and mesoscale structures of the equatorial plasma depletion. Our results also suggest that atmospheric gravity waves originating from tropospheric convection activity could play a significant seeding role in the development of equatorial plasma bubbles.Key PointsCombined GOLD/UV spectrograph images and groundâ based TEC data revealed EPB features and development over a large geographic areaBottomside F layer oscillations and traveling ionospheric disturbance were observed by ionosonde and detrended TEC resultsAtmospheric gravity waves likely play an important role in seeding the Râ T instability and the development of this EPB eventPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153570/1/jgra55456_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153570/2/jgra55456.pd