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Irregular variations


Besides regular temporal behaviour, on a magnetogram can be observed irregular features typical of perturbed conditions. One of the consequences of the complex dynamical processes occurring on the Sun is the emission of a large number of charged particles that approaching the Earth planet have the effect of perturbing the geomagnetic field recorded on the surface of the planet. The irregularity of this kind of perturbation come from the irregularity of associated phenomena on the Sun.


Magnetic storms

The magnetic storms are multi-faceted phenomena that originate at the Sun and occur in the solar wind, the magnetosphere, the ionosphere and the thermosphere.
The primary cause of geomagnetic storms is a strong enhanced transfer of energy from solar wind to the magnetosphere. Conditions for enhanced energy transfer are particularly favourable when the solar wind velocity is high and the Bz component of Interplanetary Magnetic Field (IMF) assumes negative values. Indeed, under these conditions the Earth loses its natural magnetic screen against the solar wind, thus becoming an open system for the solar wind.

The plot shows the typical trend of the horizontal component (H) of the geomagnetic field as recorded at L’Aquila (AQU) geomagnetic observatory during a geomagnetic storm (from 5/11/2001 to 13/11/2001).

At middle and low latitudes many storms display a similar general appearance for the H component of the field. Indeed, during a magnetic storm this component is reduced from 50 to 500 nT and this reduction occurs everywhere on the Earth.
A typical magnetic storm is characterized by three different phases.
The initial phase is manifested by a sudden increase in the H component (Storm Sudden Commencement). It is due to the arrival of the high dynamic pressure that compresses the magnetosphere. The size of the SSC ranges from a few nT to over 100 nT and the increase may last many hours. Not all storms have sudden commencements, storms without a SSC are called “gradual onset” storms.
The initial phase is followed, sometime later, by the main phase (or growth phase). It is characterized by a depression in the horizontal component of the magnetic field. This depression develops over a time span of one to few hours.
The magnetic storm concludes with a recovery phase that lasts many days. During this phase the H component gradually returns to its undisturbed initial level.
Typically, a storm takes about half a day to develop, and it gradually decays over the next few days.
Maps of the spatial distribution of the storm disturbance field during the main phase indicate that it is uniform over the entire Earth and it is direct parallel to the Earth’s dipole axis. Such a disturbance is produced by a current flowing around the Earth called ring current.
It has been shown, indeed, that the intensity of the magnetic disturbance observed at Earth is proportional to the total energy of this current. A measure of the strength of the equatorial ring current is given by the Dst index, which has been consequently adopted as a measure of magnetic storm severity.

 

Real-time recordings of the elements of the geomagnetic field made at the three Italian permanent observatories are available: Castello Tesino (TN), L’Aquila, and Lampedusa.

 


Magnetic substorms

A substorm is an ordered sequence of events that occurs in the magnetosphere and ionosphere when the interplanetary magnetic field (IMF) turns southward and increased energy flows from the solar wind into the magnetosphere.

Typical configuration of IMF that permits to the energy to flow from the solar wind to the magnetosphere.

On Earth the most visible sign of a substorm is a great increase of polar auroras in the midnight auroral zone. At ordinary times, quiescent auroral arcs are often seen there, but following the onset of a substorm, they intensify, move rapidly (mostly poleward) and expand, until they may cover much of the sky. Their activity may build up for half an hour and then decay, but as with atmospheric weather, patterns are quite variable. Large magnetic disturbances are also observed, up to 1000 nT (nanoTesla) which is about 2% of the total field in the auroral zone. These disturbances are considerably more intense than at low latitudes where the disturbance associated to magnetic storms may only reach 100 nT. This different intensity is due to a different distance from the Earth of the sources that are responsible of these disturbances. The ring current, responsible of the magnetic storm, circles the Earth at distances of tens of thousands of kilometers while the electric currents associated with the substorm come down to the ionosphere, only about 130 km above the ground.

Since the existence of magnetospheric substorms was first recognized there have been numerous attempts to explain what causes them. Different models have been proposed to explain this phenomenon. The model which has received the most attention is the NENL (Near Earth Neutral Line) model.
The NENL model is a phenomenological model developed from numerous studies of the plasma and magnetic field within the tail.
According to this model an isolated substorm begins when the IMF turns southward and dayside reconnection begins. For about an hour afterward, the eastward and westward electrojets, flowing from noon toward midnight along the ovals, gradually increase in strength and move equatorward along the aurora simultaneously, bands of quiet auroral arcs drift equatorward near midnight in the northern and southern auroral oval.
This phase is called the growth phase of the substorm and it is also characterized by a storage of the energy coming from the solar wind into the magnetotail. Successively, the energy stored in the magnetotail is impulsively released (which is called the energy unloading process) in coincidence with the field dipolarization. This phase known as expansion phase lasts about 30-60 minutes and it represents the substorm onset. The final phase of a substorm is called the recovery phase. During this phase the aurora and currents gradually drift back to their original equatorward locations as they simultaneously decrease in luminosity and strength. Provided that the IMF has turned northward in the intervening time, the recovery phase ends after approximately 90 minutes.

Sketches showing the development of the nightside tail magnetic field during the growth and expansion phases of a substorm.

Special geomagnetic indices have been introduced in order to describe the magnetic activity at polar latitudes. These indices are the AE indices.
 


Magnetic bays

At mid latitude in the geomagnetic records important irregular variations, known as bays, can be observed. These variations occur preferably in the evening and night hours and have a duration of 1-2 hours. The bays are often associated with storms, but can occur also separately; this suggests that they can be considered as the elementary units of a storm. From the observations it has been deduced that the bays are produced by electric currents flowing in the ionosphere at latitudes between 65° and 70° these currents are generated in the magnetosphere, flow in the ionosphere and come back in the ionosphere following the magnetic field lines.

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