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Abstract One of the most popular long‐term data sets of energetic particles used in, for example, long‐term radiation belt studies and in atmospheric/climate studies is perhaps the NOAA/POES (Polar Orbiting Environmental Satellites) data set, which extends nearly continuously from 1979 to present. The present study aims to construct a new homogeneous long‐term composite record of daily latitude distributions of energetic electrons based on the MEPED (Medium Energy Proton and Electron Detector) data. Part 1 of this study corrected the data for temporally varying background noise related to cosmic rays and for the drift in the orientation of satellite orbital planes. The present paper addresses the final and most severe problem for the data homogeneity, caused by the difference of telescope pointing directions in older SEM‐1 and newer SEM‐2 versions of the MEPED instrument. Because the telescope pitch angles and the electron pitch angle distribution change with latitude, the difference in SEM‐1 and SEM‐2 fluxes depends on latitude and varies from time to time. The systematic flux differences between SEM‐1 and SEM‐2 can range between a factor of 1.5 to more than an order of magnitude. Novel statistical methodology based on principal components and canonical correlation mapping is presented here to robustly transform the daily SEM‐1 electron latitude distributions into SEM‐2 level. The data from different POES satellites are then combined into a spatially and temporally homogeneous composite series, which is well suited, for example, for long‐term studies of radiation belts and precipitation related atmospheric ionization and its chemical and dynamical effects in the atmosphere/climate system. This article is a companion to Asikainen and Ruopsa (2019), https://doi.org/10.1029/2018JA026214, http://urn.fi/urn:nbn:fi-fe2019042312984.
Abstract Coronal holes (CHs) are regions in the solar corona characterized by plasma density lower than in the surrounding quiet Sun. Therefore they appear dark in images of the solar atmosphere made, e.g., in extreme ultraviolet (EUV). Identifying CHs on solar images is difficult since CH boundaries are not sharp, but typically obscured by magnetic structures of surrounding active regions. Moreover, the areas, shapes, and intensities of CHs appear differently in different wavelengths. Coronal holes have been identified both visually by experienced observers and, more recently, by automated detection methods using different techniques. In this article, we apply a recent, robust CH identification algorithm to a new set of homogenized EUV synoptic maps based on four EUV lines measured by the Solar and Heliospheric Observatory/Extreme ultraviolet Imaging Telescope (SOHO/EIT) in 1996–2018 and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) in 2010–2018 and create corresponding CH synoptic maps. We also use CHs of the hand-drawn McIntosh archive (McA) from 1973–2009 to extend the CH database to earlier times. We discuss the success of the four EUV lines to find CHs at high or low latitudes, and confirm that the combined EIT 195 Å/AIA 193 Å series applies best for both polar and low-latitude CH detection. While the polar CH detection suffers from the vantage-point limitation, the low-latitude CH areas extracted from this line correlate with the McA CH data very well. Using the simultaneous measurements between EIT and McA and EIT and AIA, we scale the different data series to the same level and form the longest uniform series of low-latitude CHs in 1973–2018. We find that, while the solar cycle maxima of low-latitude CHs in the descending phase of Solar Cycles 21–23 attain roughly similar values, the corresponding maximum during Solar Cycle 24 is reduced by a factor of two. This suggests that magnetic flux emergence is crucial for the formation of low-latitude CHs.
Abstract Synoptic maps of solar EUV intensities have been constructed for many decades in order to display the distribution of the different EUV emissions across the solar surface, with each map representing one Carrington rotation (i.e. one rotation of the Sun). This article presents a new solar EUV synoptic map dataset based on full-disk images from the Solar and Heliospheric Observatory/Extreme Ultraviolet Imaging Telescope (SOHO/EIT) and Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). In order to remove the significant and complicated drift of EIT and AIA EUV intensities due to sensor degradation, we construct the synoptic maps in standardized intensity scale. We describe a method of homogenizing the SOHO/EIT maps with SDO/AIA maps by transforming the EIT intensity histograms to AIA levels. The new maps cover the years from 1996 to 2018 with 307 SOHO/EIT and 116 SDO/AIA synoptic maps, respectively. These maps provide a systematic and homogeneous view of the entire solar surface in four EUV wavelengths, and are well suited, e.g., for studying long-term coronal hole evolution.
Abstract The most important driver of geomagnetic activity is the north–south (\(B_z\)) component of the interplanetary magnetic field (IMF), which dominates the solar wind-magnetosphere coupling and all solar wind coupling functions. While the east–west (\(B_y\)) IMF component is also included in most coupling functions, its effect is always assumed to be symmetric with respect of its sign. However, recent studies have shown that, for a fixed value of any coupling function, geomagnetic activity is stronger for \(B_y > 0\) than for \(B_y < 0\) in Northern Hemisphere winter. In Southern Hemisphere winter, the dependence on the \(B_y\) sign is reversed. In this paper, we use measurements of National Oceanic and Atmospheric Administration Polar-Orbiting Operational Environmental Satellites to show that the flux of magnetospheric electrons precipitating into the atmosphere also exhibits an explicit \(B_y\) dependence. This \(B_y\) dependence is strong in the midnight and dawn sectors where it causes a related \(B_y\) effect in the westward electrojet and geomagnetic activity by modulating ionospheric conductivity.
Abstract Energetic electron precipitation (EEP) affects the high‐latitude middle atmosphere by producing NOx compounds that destroy ozone. Earlier studies have shown that in the wintertime polar stratosphere, increased EEP enhances the westerly wind surrounding the pole, the polar vortex. This EEP effect has been found to depend on the quasi‐biennial oscillation (QBO) of equatorial winds, but the mechanism behind this modulation has so far remained unresolved. In this study we examine the atmospheric effect of EEP and its modulation by QBO using the corrected electron flux measurements by NOAA/POES satellites and the ERA‐Interim reanalysis data of zonal wind, temperature, and ozone in winter months of 1980–2016. We verify the EEP‐related strengthening of the polar vortex, warming (cooling) in the upper (lower) stratosphere and a reduction of ozone mass mixing ratio in the polar stratosphere. We also verify that the EEP effect is stronger and more significant especially in late winter, when the QBO at 30 hPa is easterly. We find here that the difference in the EEP effect between the two QBO phases is largest using a roughly 6‐month lag for QBO. We demonstrate that ozone mass mixing ratio in the lower polar stratosphere, a proxy for the strength of Brewer‐Dobson circulation, is also larger during QBO‐E than QBO‐W, with the difference maximizing when the QBO is lagged by 6 months. Our findings indicate that the modulation of the Brewer‐Dobson circulation by QBO controls how the EEP affects the polar vortex.
Abstract A sudden stratospheric warming (SSW) is a large‐scale disturbance of the wintertime stratosphere, which occurs especially in the Northern Hemisphere. Earlier studies have shown that SSW occurrence depends on atmospheric internal factors and on solar activity. We examine SSW occurrence in northern winters 1957/1958–2016/2017, considering several factors that may affect the SSW occurrence: Quasi‐Biennial Oscillation (QBO), El Niño–Southern Oscillation (ENSO), geomagnetic activity, and solar radiation. We confirm the well‐known result that SSWs occur more often in easterly QBO phase than in westerly phase. We show that this difference depends on how the QBO phase is determined. We find that the difference in SSW occurrence between easterly and westerly QBO winters strengthens (weakens) if geomagnetic activity or solar activity is low (high), or if the ENSO is in a cold (warm) phase. In easterly QBO phase significantly more SSWs occur during low geomagnetic activity than high activity.
Abstract Low altitude NOAA/POES satellites have measured energetic particles with MEPED instrument from 1979 to present. However, these measurements have suffered from a variety of instrumental and data quality problems, but recent advances in detailed understanding of the MEPED instrument have allowed us to calibrate and correct the data. Currently, this database forms the longest systematically calibrated energetic particle dataset in space physics. It has now been made available though the ESPAS Data Portal in 1/4 satellite orbit time resolution. The offered data products include average energetic proton and electron fluxes in two directional telescopes at different energy channels from two opposite local time sectors and hemispheres. The data allow one to study the particle fluxes in a variety of time scales from space weather events to space climate time scales over several solar cycles. Here we review the work done to correct and recalibrate the MEPED database and describe in detail the data products published in ESPAS Data Portal.
Abstract Geomagnetic activity is mainly driven by the southward (Bz) component of the interplanetary magnetic field (IMF), which dominates all solar wind coupling functions. Coupling functions also depend on the absolute value of the dawn-dusk (By) component of the IMF, but not on its sign. However, recent studies have shown that for a fixed level of solar wind driving, auroral electrojets in the Northern Hemisphere (NH) are stronger for By > 0 than for By < 0 during NH winter. In NH summer, the dependence on the By sign is reversed. While this By sign dependence, also called the explicit By-dependence, is very strong in the winter hemisphere, it is weak in the summer hemisphere. Moreover, the explicit By-dependence is much stronger in the westward electrojet than in the eastward electrojet. In this study, we study how the explicit IMF By-dependence is coupled with large-scale field-aligned currents (FACs) by using FAC measurements from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and an empirical ionospheric conductance model. We model the complete ionospheric electrodynamics by solving the current continuity equation, and show that during periods of elevated solar wind driving (Bz < 0), the IMF By component modulates Regions 1 and 2 FACs in the dawn sector of the winter hemisphere. This leads to an explicit By-dependence in ionospheric conductance and the westward electrojet. We also show that the By-dependence of FACs and conductance is weak in the dusk sector, which explains the earlier observation of the weak By-dependence of the eastward electrojet.
Abstract The dynamics of substorms are known to be dominated by the North-South (Bz) component of the Interplanetary Magnetic Field (IMF), which is the most important driver of the dayside reconnection. Even though the dawn-dusk (By) component is also known to play a role in substorm dynamics, its effects are not yet fully understood. In this paper we study how IMF By modulates the onset latitude, strength and occurrence frequency of substorms as well as the isotropic boundary (IB) latitude of energetic protons. We show that the substorm onset latitude and the IB latitude are about one degree lower for large magnitude By (|By| < 3nT) than for small By. In contrast, the substorm occurrence frequency is larger for small |By|. We suggest that the magnetotail is more stable during large |By|, requiring the magnetotail lobes (and hence the polar cap) to contain more flux to initiate a substorm compared to the situation when By is small.
Abstract One of the most popular long‐term data sets of energetic particles used in, for example, long‐term radiation belt studies and in atmospheric/climate studies is perhaps the National Oceanic and Atmospheric Administration/Polar Orbiting Environmental Satellites (POES) data set, which extends nearly continuously from 1979 to present. The energetic particle measurements by the Medium Energy Proton and Electron Detector instrument onboard the POES satellites have had many instrumental problems, which have made quantitative estimates of energetic particle fluxes somewhat difficult. However, in the recent years, these instrumental deficiencies have been studied and corrected. Here we aim to construct a new long‐term composite record of energetic electrons based on the Medium Energy Proton and Electron Detector data. In this study we point out that there are also other remaining factors, not related to instrument construction, which still severely impact the overall homogeneity of the 39‐year POES data set. We concentrate here on studying and correcting two issues: (1) temporally varying background noise related to cosmic rays and (2) drift in the orientation of satellite orbital planes, which changes the sampling location of the satellites over time. In particular, we show that the drift of satellite orbital planes leads to rather large changes in the electron fluxes over time, which could be misinterpreted as true temporal changes without the corrections. These changes can be rather large, a factor of 3 or more in the poleward edge of the precipitation zone, and are likely to have a large impact, for example, on atmospheric ionization estimates based on POES data. This article is a companion to Asikainen, T. ( 2019). New homogeneous composite of energetic electron fluxes from POES: 2. Intercalibration of SEM‐1 and SEM‐2. Journal of Geophysical Research: Space Physics, 124. https://doi.org/10.1029/2019JA026699, http://urn.fi/urn:nbn:fi-fe2019092630047.
Abstract We study the occurrence of magnetic storms in space age (1957–2021) using Dst and Dxt indices. We find 2,526/2,743 magnetic storms in the Dxt/Dst index, out of which 45% are weak, 40% moderate, 12% intense and 3% major storms. Occurrence of storms in space age follows the slow decrease of sunspot activity and the related change in solar magnetic structure. We quantify the sunspot—coronal mass ejection (CME) storm relation in the five cycles of space age. We explain how the varying solar activity changes the structure of the heliospheric current sheet (HCS), and how this affects the high-speed solar wind stream (HSS)/corotating interaction region (CIR) storms. Space age started with a record number of storms in 1957–1960, with roughly one storm per week. Solar polar fields attained their maximum in cycle 22, which led to an exceptionally thin HCS, and a space age record of large HSS/CIR storms in 1990s. In the minimum of cycle 23, for the only time in space age, CME storm occurrence reduced below that predicted by sunspots. Weak sunspot activity since cycle 23 has weakened solar polar fields and widened the HCS, which has decreased the occurrence of large and moderate HSS/CIR storms. Because of a wide HCS, the Earth has spent 50% of its time in slow solar wind since cycle 23. The wide HCS has also made large and moderate HSS/CIR storms occur in the early declining phase in recent cycles, while in the more active cycles 20–22 they occurred in the late declining phase.
Abstract Certain limitations that have been identified in existing ionospheric prediction capabilities indicate that the deeper understanding and the accurate formulation of the ionospheric response to external forcing remain always high priority tasks for the research community. In this respect, this paper attempts an investigation of the long-term behavior of the ionospheric disturbances from the solar minimum between the solar cycles 23 and 24 up to the solar maximum of solar cycle 24. The analysis is based on observations of the foF2 critical frequency and the hmF2 peak electron density height obtained in the European region, records of the Dst and AE indices, as well as measurements of energetic particle fluxes from NOAA/POES satellites fleet. The discussion of the ionospheric behavior in a wide range of geophysical conditions within the same solar cycle facilitates the determination of general trends in the ionospheric response to different faces of space weather driving. According to the evidence, the disturbances in the peak electron density reflect mainly the impact of geoeffective solar wind structures on the Earth’s ionosphere. The intensity of the disturbances may be significant (greater than 20% with respect to normal conditions) in all cases, but the ionospheric response tends to have different characteristics between solar minimum and solar maximum conditions. In particular, in contrast to the situation in solar maximum, in solar minimum years the solar wind impact on the Earth’s ionosphere is mainly built on the occurrence of ionization increases, which appear more frequent and intense than ionization depletions. The ionization enhancements are apparent in all local time sectors, but they peak in the afternoon hours, while a significant part of them seems not related with an F2 layer uplifting. Taking into account the main interplanetary drivers of the disturbances in each case, i.e. high speed streams (HSSs) and corotating interaction regions (CIRs) in solar minimum and coronal mass ejections (CME) in solar maximum, we argue that the identified tendency may be considered as evidence of the ionospheric response to different solar wind drivers.
Abstract Northern Hemisphere winter circulation is affected by both solar and terrestrial forcings. El-Niño events and volcanic eruptions have been shown to produce a negative and a positive North Atlantic Oscillation (NAO) signature, respectively. Recent studies show a positive NAO signature related to both geomagnetic activity (proxy for solar wind driven particle precipitation) and sunspot activity (proxy for solar irradiance). Here the relative role of these four different drivers on the Northern Hemisphere wintertime circulation is studied using a statistical analysis of observational and reanalysis data during 1868–2014. The phase of the Quasi-Biennial Oscillation (QBO) is used to study driver signals in different stratospheric conditions. Moreover, the effects are separated for early/mid- and late winter. Our findings suggest a stratospheric mediation of the ENSO signal to the Atlantic side, which is delayed and modulated by the QBO unlike the signal in the Pacific side. The positive NAO by volcanic activity is preferentially obtained in the westerly QBO. We also find a substantial QBO modulation for geomagnetic activity and late winter sunspot activity, which favours a stratospheric pathway and the top-down mechanisms. However, signal in the North Pacific produced by early/mid-winter sunspot activity remain rather similar in different QBO phases and supports a direct forcing from the troposphere by the bottom-up sunspot mechanism.
Abstract Energetic electron precipitation (EEP) forms ozone-depleting nitrogen and hydrogen oxides in the high-latitude middle and upper atmosphere, leading to ozone destruction and temperature enhancement that can strengthen the winter polar vortex. This EEP effect on polar vortex depends on quasi-biennial oscillation and sudden stratospheric warmings, which earlier studies relate to planetary waves. We study here the possible modulation of the EEP effect by planetary waves. We perform a principal component analysis of the vertical component of Eliassen-Palm flux (EP flux) to examine the latitudinal pattern of planetary waves. We use a multilinear regression analysis to estimate the responses of zonal wind and EP flux divergence to EEP in the northern winter stratosphere by keeping the two leading principal components of vertical EP flux as controlling factors. We find that the EEP strengthens the polar vortex most systematically when planetary wave propagation is enhanced at mid-latitudes but reduced at polar latitudes.
Abstract Precipitating auroral and radiation belt electrons are considered an important part of the natural forcing of the climate system. Recent studies suggest that this forcing is underestimated in current chemistry-climate models. The High Energy Particle Precipitation in the Atmosphere III intercomparison experiment is a collective effort to address this point. Here, eight different estimates of medium energy electron (MEE) (gt; 30 kev) ionization rates are assessed during a geomagnetic active period in April 2010. The objective is to understand the potential uncertainty related to the MEE energy input. The ionization rates are all based on the Medium Energy Proton and Electron Detector (MEPED) on board the NOAA/POES and EUMETSAT/MetOp spacecraft series. However, different data handling, ionization rate calculations, and background atmospheres result in a wide range of mesospheric electron ionization rates. Although the eight data sets agree well in terms of the temporal variability, they differ by about an order of magnitude in ionization rate strength both during geomagnetic quiet and disturbed periods. The largest spread is found in the aftermath of enhanced geomagnetic activity. Furthermore, governed by different energy limits, the atmospheric penetration depth varies, and some differences related to latitudinal coverage are also evident. The mesospheric NO densities simulated with the Whole Atmospheric Community Climate Model driven by highest and lowest ionization rates differ by more than a factor of eight. In a follow-up study, the atmospheric responses are simulated in four chemistry-climate models (CCM) and compared to satellite observations, considering both the CCM structure and the ionization forcing.
Abstract Precipitating auroral and radiation belt electrons are considered to play an important part in the natural forcing of the middle atmosphere with a possible impact on the climate system. Recent studies suggest that this forcing is underestimated in current chemistry-climate models. The HEPPA III intercomparison experiment is a collective effort to address this point. In this study, we apply electron ionization rates from three data-sets in four chemistry-climate models during a geomagnetically active period in April 2010. Results are evaluated by comparison with observations of nitric oxide (NO) in the mesosphere and lower thermosphere. Differences between the ionization rate data-sets have been assessed in a companion study. In the lower thermosphere, NO densities differ by up to one order of magnitude between models using the same ionization rate data-sets due to differences in the treatment of NO formation, model climatology, and model top height. However, a good agreement in the spatial and temporal variability of NO with observations lends confidence that the electron ionization is represented well above 80 km. In the mesosphere, the averages of model results from all chemistry-climate models differ consistently with the differences in the ionization-rate data-sets, but are within the spread of the observations, so no clear assessment on their comparative validity can be provided. However, observed enhanced amounts of NO in the mid-mesosphere below 70 km suggest a relevant contribution of the high-energy tail of the electron distribution to the hemispheric NO budget during and after the geomagnetic storm on April 6.