Negative Storm Research Articles

Positive and negative storm effects on the electron density profile at low solar activity

Positive and negative storm effects on the electron density profile at low solar activity

On the seasonal response of the thermosphere and ionosphere to geomagnetic storms

Ionosonde observations have provided the data to build a picture of the response of the midlatitude ionosphere to a geomagnetic storm. The particular characteristic of interest is the preference for “negative storms” (decrease in the peak electron density, NmF2) in summer and “positive storms” (increase in NmF2) in winter. A three‐dimensional, time‐dependent model of the coupled thermosphere and ionosphere is used to explain this dependence. During the driven phase of a geomagnetic storm the two main magnetospheric energy sources to the upper atmosphere (auroral precipitation and convective electric field) increase dramatically. Auroral precipitation increases the ion density and conductivity of the upper atmosphere; the electric field drives the ionosphere and, through collisions, forces the thermosphere into motion and then deposits heat via Joule dissipation. The global wind response is divergent at high latitudes in both hemispheres. Vertical winds are driven by the divergent wind field and carry molecule‐rich air to higher levels. Once created, the “composition bulge” of increased mean molecular mass is transported by both the storm‐induced and background wind fields. The storm winds imposed on the background circulation do not have a strong seasonal dependence, and this is not necessary to explain the observations. Numerical computations suggest that the prevailing summer‐to‐winter circulation at solstice transports the molecule‐rich gas to mid and low latitudes in the summer hemisphere over the day or two following the storm. In the winter hemisphere, poleward winds restrict the equatorward movement of composition. The altered neutral‐chemical environment in summer subsequently depletes the F region midlatitude ionosphere to produce a “negative storm”. In winter midlatitudes a decrease in molecular species, associated with downwelling, persists and produces the characteristic “positive storm”.

The role of vibrationally excited nitrogen in the formation of the mid-latitude negative ionospheric storms

Abstract. Millstone Hill ionospheric storm time measurements of the electron density and temperature during the ionospheric storms (15-16 June 1965; 29-30 September 1969 and 17-18 August 1970) are compared with model results. The model of the Earth's ionosphere and plasmasphere includes interhemispheric coupling, the H+, O+(4S), O+(2D), O+(2P), NO+, O+2 and N+2 ions, electrons, photoelectrons, the electron and ion temperature, vibrationally excited N2 and the components of thermospheric wind. In order to model the electron temperature at the time of the 16 June 1965 negative storm, the heating rate of the electron gas by photoelectrons in the energy balance equation was multiplied by the factors 5-30 at he altitude above 700 km for the period 4.50-12.00 LT, 16 June 1965. The [O]/[N2] MSIS-86 decrease and vibrationally excited N2 effects are enough to account for the electron density depressions at Millstone Hill during the three storms. The factor of 2 (for 27-30 September 1969 magnetic storm) and the & actor 2.7 (for 16-18 August 1970 magnetic storm) reduction in the daytime peak density due to enhanced vibrationally excited N2 is brought about by the increase in the O++N2 rate factor.

Response of the thermosphere and ionosphere to geomagnetic storms

Four numerical simulations have been performed, at equinox, using a coupled thermosphere‐ionosphere model, to illustrate the response of the upper atmosphere to geomagnetic storms. The storms are characterized by an increase in magnetospheric energy input at high latitude for a 12‐hour period; each storm commences at a different universal time (UT). The initial response at high latitude is that Joule heating raises the temperature of the upper thermosphere and ion drag drives high‐velocity neutral winds. The heat source drives a global wind surge, from both polar regions, which propagates to low latitudes and into the opposite hemisphere. The surge has the character of a large‐scale gravity wave with a phase speed of about 600 m s−1. Behind the surge a global circulation of magnitude 100 m s −1 is established at middle latitudes, indicating that the wave and the onset of global circulation are manifestations of the same phenomena. A dominant feature of the response is the penetration of the surge into the opposite hemisphere where it drives poleward winds for a few hours. The global wind surge has a preference for the night sector and for the longitude of the magnetic pole and therefore depends on the UT start time of the storm. A second phase of the meridional circulation develops after the wave interaction but is also restricted, in this case by the buildup of zonal winds via the Coriolis interaction. Conservation of angular momentum may limit the buildup of zonal wind in extreme cases. The divergent wind field drives upwelling and composition change on both height and pressure surfaces. The composition bulge responds to both the background and the storm‐induced horizontal winds; it does not simply rotate with Earth. During the storm the disturbance wind modulates the location of the bulge; during the recovery the background winds induce a diurnal variation in its position. Equatorward winds in sunlight produce positive ionospheric changes during the main driving phase of the storm. Negative ionospheric phases are caused by increases of molecular nitrogen in regions of sunlight, the strength of which depends on longitude and the local time of the sector during the storm input. Regions of positive phase in the ionosphere persist in the recovery period due to decreases in mean molecular mass in regions of previous downwelling. Ion density changes, expressed as a ratio of disturbed to quiet values, exhibit a diurnal variation that is driven by the location of the composition bulge; this variation explains the ac component of the local time variation of the observed negative storm phase.

Neutral gas composition changes and E×B vertical plasma drift contribution to the daytime equatorial F2-region storm effects

Abstract. Theoretical model calculations along with ground-based observations from Huancayo ionosonde station and ESRO-4 gas analyzer data, were used to estimate the contribution of neutral gas composition changes and E×B vertical plasma drift to the observed F2-layer storm effects at the geomagnetic equator. Atomic oxygen concentration increase may give the main contribution to the positive NmF2 effect when drift velocity changes are small, but negative storm effects, on the other hand, are related mostly to vertical drift variations.

The role of vibrationally excited nitrogen in the formation of the mid-latitude negative ionospheric storms

Millstone Hill ionospheric storm time measurements of the electron density and temperature during the ionospheric storms (15-16 June 1965; 29-30 September 1969 and 17-18 August 1970) are compared with model results. The model of the Earth's ionosphere and plasmasphere includes interhemispheric coupling, the H+, O+(4S), O+(2D), O+(2P), NO+, O+2 and N+2 ions, electrons, photoelectrons, the electron and ion temperature, vibrationally excited N2 and the components of thermospheric wind. In order to model the electron temperature at the time of the 16 June 1965 negative storm, the heating rate of the electron gas by photoelectrons in the energy balance equation was multiplied by the factors 5-30 at he altitude above 700 km for the period 4.50-12.00 LT, 16 June 1965. The [O]/[N2] MSIS-86 decrease and vibrationally excited N2 effects are enough to account for the electron density depressions at Millstone Hill during the three storms. The factor of 2 (for 27-30 September 1969 magnetic storm) and the & actor 2.7 (for 16-18 August 1970 magnetic storm) reduction in the daytime peak density due to enhanced vibrationally excited N2 is brought about by the increase in the O++N2 rate factor.

Modelling of composition changes during F-region storms: a reassessment

A recalculation of the global changes of thermospheric gas composition, resulting from strong heat inputs in the auroral ovals, shows that (contrary to some previous suggestions) widespread increases of mean molecular mass are produced at mid-latitudes, in summer and at equinox. Decreases of mean molecular mass occur at mid-latitudes in winter. Similar results are given by both the ‘UCL’ and ‘NCAR TIGCM’ three-dimensional models. The computed composition changes now seem consistent with the local time and seasonal response observed by satellites, and can broadly account for ‘negative storm effects’ in the ionospheric F2-layer at mid-latitudes.

Ionospheric storm effects at subauroral latitudes: A case study

An attempt is made to classify ionospheric storm effects at subauroral latitudes according to their presumed origin. The storm of December 7/8, 1982, serves as an example. It is investigated using ionosonde, electron content, and DE 2 satellite data. The following effects are distinguished: (1) positive storm effects caused by traveling atmospheric disturbances, (2) positive storm effects caused by changes in the large‐scale thermospheric wind circulation, (3) positive storm effects caused by the expansion of the polar ionization enhancement, (4) negative storm effects caused by perturbations of the neutral gas composition, and (5) negative storm effects caused by the equatorward displacement of the trough region.

F-Region Storms and Thermospheric Dynamics

This paper reviews the composition change theory of ionospheric F2-layer storms. Recent theoretical modelling confirms that changes of the thermospheric atomic/molecular concentration ratio, resulting from energy inputs at high latitudes, are the probable cause of “negative storm effects” (electron density depletions) at mid-latitudes, and may account for “positive storm effects” (electron density increases) in the winter hemisphere. Some outstanding problems are briefly summarized.

Modelling the response of the thermosphere and ionosphere to geomagnetic storms: Effects of a mid-latitude heat source

The UCL-Sheffield coupled thermosphere/ionosphere model has been used to assess the consequences of a heat source from ring current ion precipitation during a geomagnetic storm. Such a source might in principle cause the thermospheric upwelling needed to produce significant changes in the neutral molecular/atomic ratio, and thus in F2 layer electron density, it having been shown that auroral sources probably cannot produce such changes at mid-latitudes. Some alternative explanations of the F2 layer depletions are considered and largely discounted. The computations show that an O + source of around 4 mW/m2, as has been detected by satellites, could appreciably change the molecular/atomic ratio, notably in the early morning sector. The response qualitatively agrees with a number of satellite observations of composition and airglow, and with observations of NmF2. If increased in duration and intensity, the source may be able to account for the negative storm phenomena.

Bootstrap confidence intervals for design storms from exceedance series

Abstract The design quantiles of storms are derived directly in terms of design life and the risk of occurrence. Estimators for these quantiles based on exceedance series are discussed for the cases of Poisson and negative binomial storm arrival processes. The use of Bootstrap methods to estimate confidence intervals for the design quantiles is demonstrated.

A comparison of northern and southern hemisphere TEC storm behaviour

Changes in total electron content during magnetic storms are compared at stations with similar geographic and geomagnetic latitudes and eastward declinations in the northern and southern hemispheres. Mean patterns are obtained from 58 storms at ±35° and 28 storms at ± 20° latitude. The positive storm phase is generally larger (and earlier) in the southern hemisphere, while negative storm effects are larger in the north. These changes reduce the normal asymmetry in TEC between the two hemispheres. Composition changes calculated from the MSIS86 atmospheric model agree well with the maximum decreases in TEC in both seasons (when changes in the F-layer height are ignored). Recovery occurs with a time constant of about 35 h; this is 50% longer than in the MSIS86 model. There is a marked diurnal variation at 35°S, with a rapid overnight decay and enhanced values of TEC in the afternoon. This pattern is inverted (and weaker) at 35°N, where night-time decay is consistently slower than on undisturbed nights. These results require a diurnal change in composition of opposite sign in the two hemispheres, or enhanced westward winds at night changing to eastward near sunrise. There is some evidence for both these mechanisms. Following a night-time sudden commencement there is a large annual effect with daytime TEC increasing for storms near the June solstice and decreasing near December. Storms occurring between November and April tend to give large, irregular increases in TEC for several days, particularly at low latitudes. In summer and winter at both stations, the mean size of the negative phase does not increase for storms with K p > 6. The size of the positive phase is proportional to the size of the change in a p in winter, while in summer a positive phase is seen only for the larger storms.

F-layer storms and thermospheric composition

This paper discusses the present understanding of ionospheric storms, in particular storm effects in the F-layer and their relationship to changes of thermospheric composition. After summarizing the main effects of storms on the F-layer and the neutral thermosphere, the paper discusses some recent theoretical modelling of how the thermosphere responds to energy inputs at high latitudes, such as occur during storms. The high-latitude inputs set up a thermospheric "storm circulation", with consequent world-wide temperature increases and changes in chemical composition. These composition changes appear quite differently when viewed at fixed heights and at fixed pressure-levels. At fixed heights in the F-layer, there are marked increases of neutral molecular/atomic concentration ratio; but at fixed pressure-levels the increases are confined to high latitudes, with decreases at middle and low latitudes.The results of the modelling cast doubt on the theory that negative F-layer storms at midlatitudes are caused by decreases in the atomic/molecular ratio brought about by dynamical processes originating at high latitudes. The implications for storm theories, and alternative mechanisms for producing negative storm effects, are discussed. It is still considered that the negative storm effects are probably due to photochemical processes. The possibilities include widespread enhancement of the vibrational excitation of molecular nitrogen, induced by soft particle precipitation, or substantial energy inputs at latitudes well equatorward of the auroral ovals.

Modelling of thermospheric composition changes caused by a severe magnetic storm

The UCL 3-dimensional time-dependent thermospheric model, with atomic and molecular components, is used to study composition changes in the neutral gas at F-layer heights produced by a severe magnetic storm. The computations give the mean molecular weight ( MW), temperature and winds as functions of latitude, longitude, height and time for a period of 30 h. Starting from quiet-day conditions, the simulation starts with a 6-h “substorm” period in which strong electric fields are imposed in the auroral ovals, accompanied by particle input. Weaker electric fields are imposed for the remaining 24 h of the simulation. The energy input causes upwelling of air in the northern and southern auroral ovals, accompanied by localized composition changes (increases of MW), which spread no more than a few hundred kilometres from the energy sources. There is a corresponding downward settling of air at winter midlatitudes and low latitudes, producing widespread decreases of MW at a fixed pressure-level. These storm effects are superimposed on the quiet-day summer-to-winter circulation, in which upwelling occurs in the summer hemisphere and down welling in the winter hemisphere. The composition changes seen at a fixed height differ somewhat from those at a fixed pressure-level, because of the expansion resulting from the storm heating. The results can be related to the well-known prevalence of “negative” F-layer storms (with decreases of F2-layer electron density) in summer, and “positive” F-layer storms in winter and at low latitudes. However, the modelled composition changes are not propagated far enough to account for the observed occurrence of negative storms at some distance from the auroral ovals. This difficulty might be overcome if particle heating occurs well equatorward of the auroral ovals during magnetic storms, producing composition changes and negative storm effects at midlatitudes. Winds do not seem a likely cause of negative storm effects, but other factors (such as increases of vibrationally-excited N 2) are possibly important.

The worldwide distribution of positive ionospheric storms

Worldwide foF2 data from 42 ionosonde stations are analyzed for the period 15–18 June 1965, to study worldwide occurrences of positive ionospheric storms. The start of ionospheric storms in this period takes place in the polar region as negative storms, then the regions of negative storms spread equatorward along the geomagnetic latitude. Results of the present analysis show that there appear two types of positive storms during this period. The positive storms of the first type commence to appear in equatorward fronts of the spreading negative regions in both hemispheres. These two regions of positive storms then move equatorward in synchronism with the negative regions and merge near the magnetic equator to create the equatorial positive region. It is concluded from these characteristics that the positive storms of the first type originate from disturbances of the thermosphere. The positive storms of the second type are the evening enhancements which appear in the evening sector near the plasmapause of the American meridian in this case. According to the progress of the storm, the region of maximum evening enhancements shows equatorward drift in just the same manner as the motion of the plasmapause during the storm. Conjugate effects are also observed in the occurrences of the evening enhancements. These characteristics in occurrences indicate that the positive storms of the second type are caused by the effects of the magnetospheric processes. Among mechanisms which include the magnetospheric effects, plasma flux from the protonosphere seems most plausible as the cause of the evening enhancements from the fact that enhancements begin after the sunset in the winter hemisphere of the American meridian.

Numerical simulation of negative ionospheric storms using observed neutral composition data

A numerical simulation of negative ionospheric storm effects has been performed using actually observed neutral composition data. The agreement between calculated and observed variations in the F-region plasma density demonstrates that the magnetic storm associated changes in the neutral composition are indeed sufficient to explain negative storm effects at middle latitudes.

Seasonal variations of atmospheric-ionospheric disturbances

This study investigates seasonal changes in the morphology of atmospheric-ionospheric disturbances. They are inferred from variations in the global distribution of negative storm effects. It is found that during summer, even moderate magnetic activity may lead to the development of significant negative storm effects extending from the higher latitudes down to the subtropics. In the winter hemisphere the same magnetic activity produces only small disturbance effects which furthermore are restricted to the higher-latitude region. It is suggested that future models of the disturbed atmosphere and ionosphere should allow for these seasonal variations.

On explaining the negative phase of ionospheric storms

A qualitative model of the negative phase of ionospheric storms is presented. Only stations located within an atmospheric disturbance zone of a low O N 2 ratio will observe a depletion of ionization. The extent of this disturbance zone is determined by geomagnetic coordinates. Thus stations located in the North American and Australian sectors are more liable to observe negative storm effects. On the other hand it is determined by the asymmetric energy injection along the auroral oval. It follows that stations located in the early morning sector during enhanced substorm activity have a greater chance of observing negative storm effects than those situated in the daytime sector. Seasonal and magnetic storm induced changes in the O N 2 ratio are in phase during summer and out of phase during winter, explaining the seasonal variation of storm effects.

Magnetic storm associated changes in the electron content at low latitudes

Magnetic storm associated changes in the total electron content above Hawaii are investigated. After carefully discussing the method of analysis it is demonstrated that the typical effects seen at this low-latitude station are an initial increase followed by a decrease and are thus similar to those observed at mid-latitudes. A possible explanation for positive storm effects is electrodynamic lifting, whereas negative storm effects are almost certainly caused by changes in the neutral composition due to atmospheric disturbances originating in the polar zones.

The effects of thermospheric winds on the ionosphere at low and middle latitudes during magnetic disturbances

Thermospheric temperature changes associated with the preferential heating of the atmosphere at high latitudes during magnetic disturbances are shown to produce an additional storm-time equatorward wind of about 50 m s −1 at lower-middle latitudes. This is sufficiently large to substantially alter the normal daytime thermospheric winds at these latitudes; during equinox and winter conditions, for example, the normal poleward daytime wind is reversed. This storm-time wind opposes the poleward transport of ionization responsible for the equatorial anomaly and it will thus influence the latitudinal variation of electron concentration at lower-middle latitudes. It is suggested that the effect of the storm-time wind will be to produce negative storms during the day in summer; in winter positive storms may be expected during minor disturbances and negative storms during major disturbances.