The coupling of the ionosphere to processes from below remains an elusive and difficult problem, as rapidly changing external drivers from above mask variations related to lower atmospheric sources. ...Here we use superposition of unique circumstances, current deep solar minimum and a record‐breaking stratospheric warming event, to gain new insights into causes of ionospheric perturbations. We show large (50–150%) persistent variations in the low‐latitude ionosphere (200–1000 km) that occur several days after a sudden warming event in the high‐latitude winter stratosphere (∼30 km). We rule out solar irradiance and geomagnetic activity as explanations of the observed variation. Using a general circulation model, we interpret these observations in terms of large changes in atmospheric tides from their nonlinear interaction with planetary waves that are strengthened during sudden warmings. We anticipate that further understanding of the coupling processes with planetary waves, accentuated during the stratospheric sudden warming events, has the potential of enabling the forecast of low‐latitude ionospheric weather up to several days in advance.
Much theoretical and observational work has been devoted to studying the occurrence of F region polar cap patches in the Northern Hemisphere; considerably less work has been applied to the Southern ...Hemisphere. In recent years, the Madrigal database of mappings of total electron content (TEC) has improved in Southern Hemisphere coverage, to the point that we can now carry out a study of patch frequency and occurrence. We find that Southern Hemisphere patch occurrence is very similar to that of the Northern Hemisphere with a half‐year offset, plus an offset in universal time of approximately 12 hr. This is further supported by running an ionospheric model for both hemispheres and applying the same patch‐to‐background technique. Further, we present a simple physical mechanism involving a sunlit dayside plasma source concurrent with a dark polar cap, which yields a patch‐to‐background pattern very much like that seen in the TEC mappings for both hemispheres.
Plain Language Summary
The tongue of ionization (TOI) is a stream of high‐density plasma flowing antisunward across the polar cap; it may be present as one continuous structure, or it may be fragmented into discrete density patches. There is a well‐studied climatology of when the TOI (and patches) may appear in the Northern Hemisphere polar cap; it has been harder to study this issue for the Southern Hemisphere because observational data there has been comparatively scarce but has improved considerably in recent years. We carry out a systematic search for TOIs and patches in both hemispheres throughout the years 2015–2018 and find that the pattern of patch occurrence in the Southern Hemisphere is very much like that in the Northern Hemisphere, but is seasonally shifted by 6 months and diurnally shifted by 12 hr. This finding, which is based on the use of two‐dimensional grids of observations of total electron content, is very different from results reported in studies that have been based on the use of one‐dimensional satellite tracks.
Key Points
Ionospheric data coverage in the Southern Hemisphere is now comprehensive enough to allow a multiyear survey of polar cap density patches
The seasonal and diurnal distributions of patches in the SH are like those of the NH, but are shifted by 6 months and 12 hr
Our results differ in very significant ways from what has been reported in several recent studies using other methodologies
We investigate the ionospheric response to several stratospheric sudden warming events which occurred in Northern Hemisphere winters of 2008 and 2009 during solar minimum conditions. We use GPS total ...electron content data in a broad latitudinal region at ±40° geographic latitude and a single longitude, 75°W. In all cases, we find a strong daytime ionospheric response to stratospheric sudden warmings. This response is characterized by a semidiurnal character, large amplitude, and persistence of perturbations for up to 3 weeks after the peak in high‐latitude stratospheric temperatures. The ionospheric perturbations at the lower latitudes usually begin a few days after the peak in stratospheric temperature and are observed as an enhancement of the equatorial ionization anomaly (EIA) in the morning sector and a suppression of the EIA in the afternoon sector. There is also evidence of a secondary enhancement in the postsunset hours. Once observed in the low latitudes, the phase of semidiurnal perturbations progressively shifts to later local times in subsequent days. This progressive shift occurs at a different rate for different stratospheric warming events. The large magnitude and persistence of ionospheric perturbations, together with the predictability of stratospheric sudden warmings several days in advance, present an opportunity to investigate these phenomena in a systematic manner which may eventually lead to a multiday forecast of low‐latitude ionosphere conditions.
Nighttime and daytime medium‐scale traveling ionospheric disturbances (MSTIDs) are detected with dense and wide detrended total electron content (TEC) maps over North America using multiple GPS ...receiver networks. The TEC maps cover a wide region of 60–130°W and 24–54°N (30–65°N in geomagnetic latitude), and have a spatial resolution of 1.05° × 1.05° in latitude and longitude (0.15° × 0.15° with 7 × 7 pixel smoothing) and a temporal resolution of 30 seconds. The TEC maps reveal, for the first time, that the nighttime MSTIDs propagate southwestward with 200–500 km wavelengths over North America and have wavefronts longer than ∼2,000 km. We also observe that daytime MSTIDs with 300–1,000 km wavelengths propagate southeastward until mid‐afternoon and southwestward in the late afternoon. In the mid‐to‐late afternoon, these MSTIDs propagating in the different directions are superimposed. The TEC maps can be a new powerful tool to investigate the MSTIDs.
The recent discovery of large ionospheric disturbances associated with sudden stratospheric warmings (SSW) has challenged the current understanding of mechanisms coupling the stratosphere and ...ionosphere. Non‐linear interaction of planetary waves and tides has been invoked as a primary mechanism for such coupling. Here we show that planetary waves may play a more complex role than previously thought. Planetary wave forcing induces a global circulation that leads to the build‐up of ozone density in the tropics at 30–50 km altitude, the primary region responsible for the generation of the migrating semidiurnal tide. The increase in the ozone density reaches 25% and lasts for ∼35 days following the SSW, long after the collapse of the planetary waves. Ozone enhancements are not only associated with SSW but are also observed after other amplifications in planetary waves. In addition, the longitudinal distribution of the ozone becomes strongly asymmetric, potentially leading to the generation of non‐migrating semidiurnal tides. We report a persistent increase in the variability of ionospheric total electron content that coincides with the increase in stratospheric ozone and we suggest that the ozone fluctuations affect the ionosphere through the modified tidal forcing.
Key Points
Planetary waves cause increase in stratospheric ozone in the tropics
Amplified tides increase the variability in ionospheric electron density
This mechanism works not only during SSW, but also in minor warmings
During geomagnetic storms, the ionospheric F region electron density may be greatly increased or decreased, which are termed positive storms or negative storms. It is generally accepted that negative ...storm phases are caused by neutral composition changes. In contrast, different mechanisms have been proposed to explain the generation of positive storm phases. In this paper, we present observations of a strong positive storm at middle latitudes. The Millstone Hill incoherent scatter radar detected significant increases of the midlatitude ionospheric F region electron density during a magnetic storm on 3 April 2004. The positive phase of the ionospheric storm started to occur in the morning sector (0912 LT at Millstone Hill) and lasted for more than 10 hours. Compared with the quiet‐time ionosphere, the daytime F peak altitude over Millstone Hill during the storm was ∼80 km higher, the F region electron density was increased by a factor of 2–4, and the F region electron temperature was decreased by ∼1000 K (or ∼40%). The radar also measured an enhanced eastward electric field and a slightly more equatorward neutral wind. Global GPS measurements show that total electron content (TEC) was increased at middle latitudes and decreased at lower latitudes, and the large TEC enhancements occurred primarily in the Atlantic sector. We suggest that electric fields may play an important role in the generation of the observed positive storm phase. The eastward electric field will cause increases in the midlatitude ionospheric electron density by moving the plasma particles upward and decreases in the equatorial ionospheric electron density by strengthening the fountain effect. The daytime poleward wind was reduced or even reversed during the storm, which may also contribute to the occurrence of the positive storm.
We examine the relationship of convection electric fields to the formation of a polar cap tongue of ionization (TOI) from midlatitude plumes of storm enhanced density (SED). Observations from the ...geomagnetic storm on 26–27 September 2011 are presented for two distinct SED events. During an hour‐long period of geomagnetic activity driven by a coronal mass ejection, a channel of high‐density F region plasma was transported from the dayside subauroral ionosphere and into the polar cap by enhanced convection electric fields extending to middle latitudes. This TOI feature was associated with enhanced HF backscatter, indicating that it was the seat of active formation of small‐scale irregularities. After the solar wind interplanetary magnetic field conditions quieted and the dayside convection electric fields retreated to higher latitudes, an SED plume was observed extending to, but not entering, the dayside cusp region. This prominent feature in the distribution of total electron content (TEC) persisted for several hours and elongated in magnetic local time with the rotation of the Earth. No ionospheric scatter from SuperDARN radars was observed within this SED region. The source mechanism (enhanced electric fields) previously drawing the plasma from midlatitudes and into the polar cap as a TOI was no longer active, resulting in a fossil feature. We thus demonstrate the controlling role exercised by the convection electric field in generating a TOI from midlatitude SED.
Key Points
Present observations of ionospheric electric field controlling SED/TOI formationAnalyze extent of small‐scale irregularity backscatter within high‐TEC featuresDiscuss characteristics of fossil SED feature which persists for several hours
We present a global view of large‐scale ionospheric disturbances during the main phase of a major geomagnetic storm. We find that the low‐latitude, auroral, and polar latitude regions are coupled by ...processes that redistribute thermal plasma throughout the system. For the large geomagnetic storm on 20 November 2003, we examine data from the high‐latitude incoherent scatter radars at Millstone Hill, Sondrestrom, and EISCAT Tromso, with SuperDARN HF radar observations of the high‐latitude convection pattern and DMSP observations of in situ plasma parameters in the topside ionosphere. We combine these with north polar maps of stormtime plumes of enhanced total electron content (TEC) derived from a network of GPS receivers. The polar tongue of ionization (TOI) is seen to be a continuous stream of dense cold plasma entrained in the global convection pattern. The dayside source of the TOI is the plume of storm enhanced density (SED) transported from low latitudes in the postnoon sector by the subauroral disturbance electric field. Convection carries this material through the dayside cusp and across the polar cap to the nightside where the auroral F region is significantly enhanced by the SED material. The three incoherent scatter radars provided full altitude profiles of plasma density, temperatures, and vertical velocity as the TOI plume crossed their different positions, under the cusp, in the center of the polar cap, and at the midnight oval/polar cap boundary. Greatly elevated F peak density (>1.5E12 m−3) and low electron and ion temperatures (∼2500 K at the F peak altitude) characterize the SED/TOI plasma observed at all points along its high‐latitude trajectory. For this event, SED/TOI F region TEC (150–1000 km) was ∼50 TECu both in the cusp and in the center of the polar cap. Large, upward directed fluxes of O+ (>1.E14 m−2 s−1) were observed in the topside ionosphere from the SED/TOI plume within the cusp.
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.
Plain Language Summary
On 21 August 2017, during daytime hours (16:00–20:00 UTC), a total solar eclipse was observed across the continental United States. A dense network of GPS receivers was utilized to monitor the changes in the ionosphere. GPS data were analyzed for the latitudinal and longitudinal response of the total electron content and for the presence of ionospheric perturbations during eclipse passage. A significant TEC depletion, in some cases greater than 60%, was observed associated with the eclipse shadow. Large‐scale ionospheric perturbations were detected over the United States prior to and following the major TEC depletion observed near the time of totality. Signatures of enhanced TEC structures were also observed over the Rocky Mountain chain during the main period of TEC depletion.
Key Points
GNSS observations during August 2017 eclipse show total electron content (TEC) depletions up to 60% in magnitude
Enhanced large‐scale traveling ionospheric disturbances are observed before, during, and after totality
Enhanced TEC is observed above Rocky Mountains 5‐10 min after totality
We have used all‐sky imaging to relate different types of auroral oval disturbances to large‐scale traveling ionospheric disturbances (LSTIDs). We selected eight nights with good all‐sky imaging and ...Global Positioning System total electron content coverage, including five non–storm time periods with isolated initiations of geomagnetic activity and three storm main phase periods with continuous activity. Periods with LSTIDs generally started and stopped with initiation and cessation of activity. We found evidence that individual LSTIDs often show 1‐1 correspondence with identifiable auroral disturbances, disturbances either being related to a substorm onset or to auroral streamers without a substorm. Since substorm ground magnetic depressions are directly related to the electric fields and electron precipitation of auroral streamers, we hypothesize that streamers may be the primary drivers of individual nightside LSTIDs with or without a substorm. Additionally, we found evidence that (1) LSTIDs detection is more likely near the longitude range of the initiating disturbance than further away, (2) the orientation of LSTID phase fronts depends on location relative to disturbance longitude, and (3) disturbance ionospheric current and magnetic latitude may influence whether a given disturbance leads to a detectable LSTID. Numerous LSTIDs (10 to 12 over 7‐ to 8‐hr periods) were detected during southward interplanetary magnetic field periods of coronal mass ejection storm main phases, the vast majority reflecting streamers in the absence of substorms. Less LSTIDs were seen during the one examined high‐speed‐stream storm. We have also found evidence that omega band disturbances may drive interesting TIDs that are distinct from the LSTIDs driven by the substorm and streamer disturbances.
Key Points
All‐sky imaging was used to relate auroral oval disturbances to large‐scale traveling ionospheric disturbances (LSTIDs)
Streamers may be the primary drivers of individual nightside LSTIDs, with or without a substorm
Possible LSTID longitude, orientation, and strength connections suggested
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