Solar Minimum Conditions Research Articles

Qualitative and Quantitative Assessment of the SET HASDM Database

AbstractThe High Accuracy Satellite Drag Model (HASDM) is the operational thermospheric density model used by the US Space Force Combined Space Operations Center. By using real‐time data assimilation, HASDM can provide density estimates with increased accuracy over other empirical models. With historical HASDM density data being released publicly for the first time, we can analyze the data to compare dominant modes of variability in the upper atmosphere as modeled by HASDM and the Jacchia‐Bowman 2008 Empirical Thermospheric Density Model (JB2008), a Jacchia family model upon which density corrections are made as a part of the HASDM framework. This model comparison is conducted through the use of principal component analysis (PCA) which shows the increased variability of the HASDM dataset. We highlight HASDM's ability to capture the movement of lighter species during solar minimum conditions, unlike many empirical models. We then compare density from both models to the CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) accelerometer‐derived density estimates. This comparison shows that HASDM more closely matches the accelerometer‐derived densities with mean absolute differences of compared to CHAMP and GRACE‐A, respectively. The comparison also reveals improved representation of cooling mechanisms due to NO and CO2 by the HASDM database.

The Martian ionosphere at solar minimum: Empirical model validation using MAVEN ROSE data

The Radio Occultation Science Experiment (ROSE; Withers et al., 2018, 2020) on the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite has produced over 400 electron density profiles from July 2016 to November 2019. These Ne(h) profiles occurred over a large range of solar zenith angles (54o- 130o) and solar flux conditions (24–54 solar flux units at Mars). One of the goals of the MAVEN mission is to characterize the status of the topside ionosphere at Mars as a reservoir of possible escaping plasma. Here we evaluate how ROSE topside ionospheric measurements, made predominantly under solar minimum conditions, compare with the only empirical model of the topside ionosphere (Němec et al., 2019). To assess congruence between the model and the observations, a deviation factor (DF) is calculated for each predicted versus observed Ne(h) profile. Diurnally, low DFs (and thus higher agreement) occur for occultations with solar zenith angles (SZA) < 82o. On a longer time scale, lower solar fluxes tend to have higher DF values. Correlations of the maximum electron density (Nmax) of each profile (predicted and observed) have an overall correlation coefficient (CC) of 0.96. Similarly, the observed total electron content of the topside ionosphere (TTEC) agrees with predictions (CC = 0.86). The model and the predictions differ most for the altitude of peak density (CC = 0.62 for SZA < 90o).

Salient Midlatitude Ionosphere‐Thermosphere Disturbances Associated With SAPS During a Minor but Geo‐Effective Storm at Deep Solar Minimum

AbstractThis work conducts a focused study of subauroral ion‐neutral coupling processes and midlatitude ionospheric/thermospheric responses in North America during a minor but quite geo‐effective storm on September 27–28, 2019 under deep solar minimum conditions. Several prominent storm‐time disturbances and associated electrodynamics/dynamics were identified and comprehensively analyzed using Millstone Hill and Poker Flat incoherent scatter radar measurements, Fabry‐Perot interferometer data, total electron content data from Global Navigation Satellite System observations, and thermospheric composition O/N2 data from the Global‐scale Observations of Limb and Disk mission. Despite solar minimum conditions, this minor storm produced several prominent dynamic features, in particular (a) Intense subauroral polarization stream (SAPS) of 1,000 m/s, overlapping with a deepened main trough structure. (b) An enhanced westward wind of 230 m/s and a significant poleward wind surge of 85 m/s occurred in the post‐SAPS period. (c) Large‐scale traveling ionospheric disturbances (TIDs) were generated and propagated equatorward across mid‐latitudes in the storm main phase. TID characteristics were significantly affected by SAPS, evolving into divergent propagation patterns. (d) SAPS was situated on the poleward edge of a considerable storm‐enhanced density structure. (e) The midlatitude ionosphere and thermosphere exhibited a prolonged positive storm effect in the main phase and beginning of recovery phase, with 5–10 TECU increase and 10%–30% O/N2 enhancement for 12 h. This was followed by a considerable negative storm effect with 5–10 TECU and 20%–40% O/N2 decrease. Results show that minor storm intervals can produce substantial mid‐latitude ionospheric and thermospheric dynamics in low solar flux conditions.

The TSIS-1 Hybrid Solar Reference Spectrum.

We present a new solar irradiance reference spectrum representative of solar minimum conditions between solar cycles 24 and 25. The Total and Spectral Solar Irradiance Sensor‐1 (TSIS‐1) Hybrid Solar Reference Spectrum (HSRS) is developed by applying a modified spectral ratio method to normalize very high spectral resolution solar line data to the absolute irradiance scale of the TSIS‐1 Spectral Irradiance Monitor (SIM) and the CubeSat Compact SIM (CSIM). The high spectral resolution solar line data are the Air Force Geophysical Laboratory ultraviolet solar irradiance balloon observations, the ground‐based Quality Assurance of Spectral Ultraviolet Measurements In Europe Fourier transform spectrometer solar irradiance observations, the Kitt Peak National Observatory solar transmittance atlas, and the semi‐empirical Solar Pseudo‐Transmittance Spectrum atlas. The TSIS‐1 HSRS spans 202–2730 nm at 0.01 to ∼0.001 nm spectral resolution with uncertainties of 0.3% between 460 and 2365 nm and 1.3% at wavelengths outside that range.

Parker Solar Probe observations of He/H abundance variations in SEP events inside 0.5 au

Aims. The Parker Solar Probe (PSP) orbit provides an opportunity to study the inner heliosphere at distances closer to the Sun than previously possible. Due to the solar minimum conditions, the initial orbits of PSP yielded only a few solar energetic particle (SEP) events for study. Recently during the fifth orbit, at distances from 0.45 to 0.3 au, the energetic particle suite on PSP, Integrated Science Investigation of the Sun (IS⊙IS), observed a series of six SEP events, adding to the limited number of SEP events studied inside of 0.5 au. Variations in the H and He spectra and the He/H abundance ratio are examined and discussed in relation to the identified solar source regions and activity. Methods. IS⊙IS measures the energetic particle environment from ~20 keV to &gt;100 MeV/nuc. Six events were selected using the ~1 MeV proton intensities, and while small, they were sufficient to calculate proton and helium spectra from ~1 to ~10 MeV/nuc. For the three larger events, the He/H ratio as a function of energy was determined. Using the timing of the associated radio bursts, solar sources were identified for each event and the eruptions were examined in extreme ultraviolet emission. Results. The largest of the selected events has peak ~1 MeV proton intensities of 3.75 (cm2 sr s MeV)−1. Within uncertainties, the He and H spectra have similar power law forms with indices ranging from −2.3 to −3.3. For the three largest events, the He/H ratios are found to be relatively energy independent; however, the ratios differ substantially with values of 0.0033 ± 0.0013, 0.177 ± 0.047, and 0.016 ± 0.009. An additional compositional variation is evident in both the 3He and electron signatures. These variations are particularly interesting as the three larger events are likely a result of similar eruptions from the same active region.

The ionosphere of Mars from solar minimum to solar maximum: Dayside electron densities from MAVEN and Mars Global Surveyor radio occultations

Conditions in the ionosphere of Mars vary from solar minimum to solar maximum due to changes in the ionizing solar irradiance. Although changes in the main ionospheric layers, namely the M2 layer and the lower M1 layer, over the solar cycle have been well-characterized by previous work, corresponding changes in the upper regions of the photochemical ionosphere (∼140–200 km) have not. Here we investigate those changes using ionospheric electron density profiles acquired at solar zenith angles of 70°–90° under solar minimum conditions by the MAVEN Radio Occultation Science Experiment (ROSE) and under solar maximum conditions by a similar radio occultation experiment on Mars Global Surveyor. Due to the increase in ionizing irradiance from solar minimum to solar maximum, we expect that densities at all altitudes will increase from solar minimum to solar maximum. Due to the eccentricity of Mars’s orbit, observed electron density profiles must be adjusted to a common Mars–Sun distance to highlight solar cycle effects. This process involves increasing observed electron density values by a factor proportional to the Mars–Sun distance, which mitigates for variations in incident irradiance, and expressing altitude relative to the main ionospheric peak, which mitigates for variations in heating and expansion of the neutral thermosphere.We find that the scale height of the upper regions of the photochemical ionosphere is remarkably constant between solar minimum and solar maximum, whereas the corresponding neutral temperature increases by 50%. Yet these two quantities are thought to be proportional. Electron density values in the upper regions of the photochemical ionosphere and peak electron density values increase similarly from solar minimum to solar maximum.

Imprints of the secondary interstellar hydrogen atoms at 1 au

ABSTRACT In this paper, we search for the possible imprints of the secondary interstellar hydrogen atoms created at the heliospheric boundary in the full-sky maps of the hydrogen fluxes at the Earth orbit. By using our three-dimensional time-dependent kinetic model, the maps of the hydrogen fluxes are calculated for different phases of the solar cycle and different energy ranges. It is shown that the flux maps obtained during the solar minimum conditions for the energy range 1–20 eV have specific features such as blobs and tails, which are pronounced due to a signal of the secondary component of the interstellar atoms. We investigate how these features depend on parameters of the secondary atoms far away from the Sun and found that the geometry and shape of the tails depend on the averaged velocity and kinetic temperatures of the secondary population. The results of the paper provide a strategy where and when we need to look in order to detect the secondary component of the interstellar hydrogen separately from the primary component at 1 au. This can be important for future space missions devoted to the exploration of the heliospheric boundary.

Review of the Particle Background of the Athena X-IFU Instrument

Abstract X-ray observations are limited by the background, due to the intrinsic faintness or diffuse nature of the sources. The future Athena X-ray observatory has among its goals the characterization of these sources. We aim at characterizing the particle-induced background of the Athena microcalorimeter, in both its low- (soft protons) and high-energy (galactic cosmic rays—GCR) induced components, to assess the instrument capability to characterize background-dominated sources such as the outskirts of clusters of galaxies. We compare two radiation environments, namely the L1 and L2 Lagrangian points, and derive indications against the latter. We estimate the particle-induced background level on the X-IFU microcalorimeter with Monte Carlo simulations, before and after all of the solutions adopted to reduce its level. Concerning the GCR-induced component, the background level is compliant with the mission requirement. Regarding the soft-proton component, the analysis does not predict dramatically different backgrounds in the L1 and L2 orbits. However, the lack of data concerning the L2 environment labels it as very weakly characterizable, and thus we advise against choosing it as the orbit for X-ray missions. We then use these background levels to simulate the observation of a typical galaxy cluster from its center out to 1.2 R 200 to probe the characterization capabilities of the instrument out to the outskirts. We find that without any background reduction, it is not possible to characterize the properties of the cluster in the outer regions. We also find no improvement in the observations when carried out during the solar maximum with respect to solar minimum conditions.

Distributions of Ion Density Irregularities and Their Gradients Inside Low‐Latitude Ionospheric Plasma Bubbles Based on C/NOFS Planar Langmuir Probe Data

AbstractData from the planar Langmuir probe onboard the Communication/Navigation Outage Forecasting System has been processed, with focus on the: (1) identification of Equatorial Plasma Bubbles located within the eclipse sections of the satellite orbits; (2) subdivision of each Equatorial Plasma Bubble into three sections (West Wall, Center, and East Wall); and (3) statistical analyses of the standard deviations and gradients of ion density data for each section. The statistical distributions resulted from the application of the above processing to structures identified during the years October 1, 2008 to September 30, 2009 (solar minimum conditions) and June 5, 2012 to June 4, 2013 (close to maximum conditions of solar cycle 24). The statistical distributions have also been classified considering all longitudes or three 60° longitude sectors in South America, Africa, India‐Asia, as well as all altitudes of the C/NOFS satellite or the limited range from 650 to 800 km. It will be shown that all the parameters (Equatorial Plasma Bubble sections, solar activity, longitude sector, and altitude range) affect the resulting statistical distributions. Additionally, the probability density functions (pdfs) of ion density gradients for the EPB West Walls, Centers, and East Walls and the South American longitude sector, detected during 2012 and estimated with different resolutions will be compared. While the pdfs based on 1‐Hz low‐resolution data (sampling large‐scale structures) display east‐west asymmetries, those based on the original 512‐Hz high‐resolution data (sampling small‐scale structures), created by secondary plasma instabilities, are essentially symmetric around the reference 0 mm/km gradient.

Data‐Driven Ensemble Modeling of Equatorial Ionospheric Electrodynamics: A Case Study During a Minor Storm Period Under Solar Minimum Conditions

AbstractThe dayside equatorial ionospheric electrodynamics exhibit strong variability driven simultaneously by highly changeable external forcings that originate from the solar extreme ultraviolet (EUV), magnetosphere, and lower atmosphere. We investigate this variability by carrying out comprehensive data‐driven ensemble modeling using a coupled model of the thermosphere and ionosphere, with the focus on the vertical E × B drift variability during a solar minimum and minor storm period. The variability of vertical E × B drift in response to the changes and uncertainty of primary forcings (i.e., solar EUV, high‐latitude plasma convection and auroral particle precipitation, and lower‐atmospheric tide and wave forcing) is investigated by ensemble forcing sensitivity experiments that incorporate data‐driven stochastic perturbations of these forcings into the model. Second, the impact of assimilating FORMOsa SATellite‐3/Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT‐3/COSMIC) electron density profiles (EDPs) on the reduction of uncertainty of the modeled vertical E × B drift variability resulting from inadequately specified external forcing is revealed. The Communication and Navigation Outage Forecasting System (C/NOFS) ion drift velocity observations are used for validation. The validation results support the importance of the use of a data‐driven forcing perturbation methods in ensemble modeling and data assimilation. In conclusion, the solar EUV dominates the global‐scale day‐to‐day variability, while the lower atmosphere tide and wave forcing is critical to determining the regional variability. The modeled vertical E × B drift is also sensitive to the magnetospheric forcing. The ensemble data assimilation of FORMOSAT‐3/COSMIC EDPs helps to reduce the uncertainty and improves agreement of the modeled vertical E × B drifts with C/NOFS observations.

Extreme Positive Ionosphere Storm Triggered by a Minor Magnetic Storm in Deep Solar Minimum Revealed by FORMOSAT‐7/COSMIC‐2 and GNSS Observations

AbstractThis study examines an unexpected and extreme positive ionospheric response to a minor magnetic storm on August 5, 2019 by using global ionosphere specification (GIS) 3D electron density profiles obtained by assimilating radio occultation total electron content (TEC) measurements of the recently launched FORMOSAT‐7/COSMIC‐2 satellites, and ground‐based global navigation satellite system (GNSS) TEC. The results reveal ∼300% enhancement of equatorial ionization anomaly (EIA) crests, appearing over 200–300 km altitudes, and a much intense localized density enhancement over the European sector. These are the most intense ionospheric response that has ever been detected for a small magnetic storm with Dst ∼ −53 nT (SYM‐H ∼ −64 nT). The enhancements are validated by using global ionosphere map (GIM) TEC and ground‐based GNSS TEC. The GIS vertical electron density structures during the storm are examined to understand the physical processes giving rise to such an intense ionosphere response during deep solar minimum conditions when the background electron density is very low. Altitude variations and poleward shifts of the locations of the EIA crests indicate that prompt penetration electric fields (PPEF) play an important role in producing the observed positive storm responses, with the storm‐induced equatorward circulation supporting the plasma accumulation against recombination losses. Additional physical mechanisms are required to fully explain the unexpected electron density enhancements for this minor storm event.

Validation of ICON-MIGHTI Thermospheric Wind Observations: 1. Nighttime Red-Line Ground-Based Fabry-Perot Interferometers.

Observations of the nighttime thermospheric wind from two ground-based Fabry-Perot Interferometers are compared to the level 2.1 and 2.2 data products from the Michelson Interferometer Global High-resolution Thermospheric Imaging (MIGHTI) onboard National Aeronautics and Space Administration's Ionospheric Connection Explorer to assess and validate the methodology used to generate measurements of neutral thermospheric winds observed by MIGHTI. We find generally good agreement between observations approximately coincident in space and time with mean differences less than 11 m/s in magnitude and standard deviations of about 20-35 m/s. These results indicate that the independent calculations of the zero-wind reference used by the different instruments do not contain strong systematic or physical biases, even though the observations were acquired during solar minimum conditions when the measured airglow intensity is weak. We argue that the slight differences in the estimated wind quantities between the two instrument types can be attributed to gradients in the airglow and thermospheric wind fields and the differing viewing geometries used by the instruments.

МОДЕЛИРОВАНИЕ РАДИАЦИОННОЙ НАГРУЗКИ НА РАДИАЦИОННЫЙ МОНИТОР RADOM НА КОСМИЧЕСКОМ АППАРАТЕ «ЧАНДРАЯН-1» ПРИ ПОЛЕТЕ В ПРЕДЕЛАХ МАГНИТОСФЕРЫ ЗЕМЛИ

Planners of a crewed mission to the Moon should have confidence in reliability of radiation dose estimations. In the absence of proton events during solar minimum the absorbed dose to the crew crossing the magnetosphere will be contributed primarily by protons and electrons of the inner and outer radiation belts of Earth, respectively. It is necessary to examine whether the existing electron forecast model is good enough for this purpose. The paper describes our efforts to model the radiation environment of RADOM onboard Chandrayaan 1 launched to the Moon on 22.10.2008, including the vehicle design in order to assess its shielding function. The geomagnetic field parameters were reproduced with the use of model А2000 developed at the Nuclear Physics Institute (Moscow State University). The absorbed doses were calculated with the help of standard Canopus-80 tools. Comparison of calculated and experimental data showed a good agreement for the period of solar minimum and quiet geomagnetic conditions.

Hydrogen and helium escape on Venus via energy transfer from hot oxygen atoms

ABSTRACT Due to the relatively strong gravity on Venus, heavy atmospheric neutrals are difficult to accelerate to the escape velocity. However, a variety of processes, such as the dissociative recombination of ionospheric O$_2^+$, are able to produce hot atoms which could deliver a significant amount of energy to light neutrals and drive their escape. In this study, we construct a Monte Carlo model to simulate atmospheric escape of three light species, H, H2, and He, on Venus via such a knock-on process. Two Venusian background atmosphere models are adopted, appropriate for solar minimum and maximum conditions. Various energy-dependent and species-dependent cross-sections, along with a common strongly forward scattering angle distribution, are used in our calculations. Our model results suggest that knock-on by hot O likely plays the dominant role in driving total atmospheric hydrogen and helium escape on Venus at the present epoch, with a significant portion contributed from regions below the exobase. Substantial variations are also revealed by our calculations. Of special interest is the modelled reduction in escape flux at high solar activities for all species, mainly associated with the enhancement in thermal O concentration near the exobase at high solar activities which hinders escape. Finally, model uncertainties due to several controlling factors, including the distribution of relevant light species in the background atmosphere, the plane-parallel approximation, and the finite O energy distribution, are evaluated.

Observation and Simulation of the Development of Equatorial Plasma Bubbles: Post‐Sunset Rise or Upwelling Growth?

AbstractThis study investigates the underlying physics of equatorial plasma bubbles (EPBs) on 11 December 2019, under solar minimum conditions. The Global‐scale Observations of the Limb and Disk (GOLD) ultraviolet nightglow images exhibit a periodic distribution of reduced emissions related to EPBs. Remarkably, FORMOSAT‐7/COSMIC‐2 (F7/C2) observes a significant altitudinal difference of ~45 km in the bottomside ionosphere between two nearly collocated electron density profiles before the onset of EPBs, indicating the presence of an upwelling. Distinct ionospheric perturbations are also observed in F7/C2 and ground‐based Global Positioning System observations, suggesting that gravity waves may contribute to the upwelling. Simulations with SAMI3/ESF are further carried out to evaluate the upwelling growth and pre‐reversal enhancement (PRE) effect on EPB development. Simulations reveal that the crests of upwellings show a localized uplift of ~50 km, and EPBs only develop from the crest of upwellings. The uplift altitude of upwellings is comparable to the F7/C2 observations and the post‐sunset rise in moderate solar conditions. The polarization electric field (Ep) developed within the upwellings can drive vertical Ep × B drifts of ~32–35 m/s, which are comparable to the PRE vertical E × B drifts. We find that the PRE alone cannot drive EPBs without upwelling growth, but it can facilitate the upwelling growth. Observations and simulations allow us to conclude that upwelling growth could play a vital role in the formation of EPBs.

Recovery and Validation of Venus Ionospheric Electron Density Profiles from Pioneer Venus Orbiter Radio Occultation Observations

Abstract The Pioneer Venus Orbiter radio occultation experiment acquired vertical profiles of electron density in the ionosphere of Venus. These profiles were not readily accessible to researchers. Here we report the recovery and validation of a set of electron density profiles from Pioneer Venus Orbiter. More than 200 profiles spanning solar zenith angles of 30°–170° were deemed acceptable for scientific analysis. These profiles sample solar minimum, moderate, and maximum conditions. The profiles are publicly available and ready for use in scientific investigations of the ionosphere of Venus and related topics.

Statistical Structure of Nighttime O2 Aurora From SABER and Its Dependence on Geomagnetic and Solar Activities in Winter

AbstractO2 aurora is one kind of important molecular aurorae that is not fully understood yet. It is hard to be investigated due to the contamination by nightglow. In this work, we studied O2 aurora based on the observations of O2 emission at 1.27 μm from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument during the nighttime over 18 years. The horizontal structure of O2 auroral zone is given for the first time, and the vertical profile of O2 auroral volume emission rate is obtained after removing O2 nightglow contamination. The O2 auroral intensity and the peak height and peak volume emission rate of the vertical profile averaged over the O2 auroral zone are studied in detail. For Kp Levels 1–5, the O2 auroral peak height varies between 104 and 112 km and is negatively correlated with the auroral intensity and peak volume emission rate. The O2 auroral intensity varies between 0.14 and 5.97 kR, and the peak volume emission rate varies between 0.97 × 102 and 41.01 × 102 photons cm−3 s−1. Furthermore, the dependences of the O2 aurora on geomagnetic and solar activities are analyzed. The O2 auroral intensity and peak volume emission rate exponentially increases with increasing Kp index, whereas the peak height decreases with increasing Kp index. The O2 auroral intensity and peak volume emission rate under solar minimum condition are larger than those under solar maximum condition. The peak height under solar minimum condition is lower than that under solar maximum condition.

Characteristics of Ionospheric Irregularities Using GNSS Scintillation Indices Measured at Jang Bogo Station, Antarctica (74.62°S, 164.22°E)

AbstractGlobal Navigation Satellite System (GNSS) signals strongly depend on the ionospheric conditions, which are composed of electrons and ions generated by solar radiation and particle precipitation. Ionospheric plasma irregularities may cause the scintillation of the GNSS signals or even the loss of signal lock, resulting in the reduction of positioning accuracy and timing precision. Phase scintillation phenomenon is known to occur frequently at high latitudes and primarily related to a significant plasma density gradient, which is due to fast plasma flows in the polar region, energetic particle precipitation in the auroral region, polar cap patches, or several instability mechanisms. Statistical studies are required to understand the characteristics of ionospheric (both phase and amplitude) scintillations at high latitudes. Here, we report the results of ionospheric scintillation measurements at Jang Bogo Station (JBS; 74.62°S, 164.22°E), located inside the polar cap region in Antarctica. The occurrence rates of ionospheric scintillations over the JBS are recorded for 2 years (2017–2018) during solar minimum conditions. The occurrence rates of amplitude scintillations increase only at lower elevation angles (below 30°), which are hard to determine whether the source is ionospheric irregularity or ambient noise such as multipath. In contrast, the occurrence rates of phase scintillations depend on the azimuth angle, season, magnetic activity, magnetic local time, and signal frequency. The results of our analysis suggest that users of the GNSS should consider these parameters to prepare for the degradation of the GNSS performance at high latitudes in the Southern Hemisphere.

A Generalized Fisk-type HMF: Implications of Spatially Dependent Photospheric Differential Rotation

Abstract The existence of a Fisk-type heliospheric magnetic field (HMF) has been debated ever since Fisk proposed an alternative to the traditional view of the HMF first proposed by Parker. Several modifications of the original Fisk field model have been published in the past, for example, the Schwadron field and the Fisk–Parker hybrid HMF model. This study presents a new generalized Fisk HMF model that implements for the first time a spatially dependent differential rotation rate of the photosphere, by mapping magnetic field lines from the solar wind source surface to the photosphere. Data analysis methods of Forsyth et al. are used to search for a signature of a Fisk-type field during solar minimum conditions, using the magnetic field data from the first solar orbit of the Ulysses spacecraft. The new generalized Fisk field agrees better with the observed magnetic field winding angle than a standard Parker field during the majority of intervals scanned by Ulysses.

Role of Bottom‐Side Density Gradient in the Development of Equatorial Plasma Bubble/Spread F Irregularities: Solar Minimum and Maximum Conditions

AbstractFrom the analysis of Digisonde data over Brazilian equatorial and low‐latitude sites, we investigate the relative importance of the different parameters driving the generation of rising bubble‐type and bottom‐type spread F (SF) irregularities. Data for the complete month of October 2001, a solar maximum epoch (F10.7 = 210), and that of October 2008, an extended solar minimum period (F10.7 = 70), are analyzed to examine the SF intensity and occurrence rate as a function of the evening prereversal vertical drift velocity and the corresponding F layer heights and the bottom‐side density gradient. While the SF at the equatorial site is indicative of both the bottom‐side irregularities and rising bubbles, the SF at the low latitude represents exclusively the latter. Comparison of the results, from the two epochs, reveals a large decrease in the intensity and occurrence rate of plasma bubbles, with a decrease in solar flux. But a notable increase in these characteristics is observed in the case of bottom‐side SF. It is found that a larger (steeper) density gradient of the F layer bottom side that exists in the low solar flux condition is responsible for an enhanced Raleigh‐Taylor instability growth, counterbalancing a reduction in this rate that may arise from a smaller prereversal vertical drift and lower layer height that also characterize the low solar flux condition. Thus, the role of the bottom‐side density gradient in the ESF instability growth has been identified for the first time in terms of its ability to explain the contrasting irregularity features as observed during solar flux maximum and minimum years.