Monthly Archives: June 2014

The Ionospheric Storm

The magnetic storm is a fascinating geophysical phenomenon, which goes far beyond the visible evidence corresponding to auroral displays at high latitudes. It is central to the issues surrounding what is now referred to as space weather. A discourse on this subject is beyond the scope of this article, but the reader is referred to an excellent geophysical monograph edited by Tsurutani et al. (34).

The ionospheric storm is the ionosphere’s response to a geomagnetic storm. While the ionospheric response to magnetic storms is varied, it has been shown that they may be conveniently classified as either positive or negative in nature. The main attribute of so-called negative storms is that they are generally associated with decreases in foF2. Positive storms exhibit the opposite behavior. At midlatitudes the ionospheric storm signature is typically commensurate with the main features of a negative storm, although variations may occur. Often the temporal (or storm-time) pattern is complex. For example, the midlatitude ionospheric response to a large magnetic storm is generally characterized by a short-lived increase in the F-region electron concentration in the dusk sector following storm commencement (SC), after which it decreases dramatically (see Fig. 13). The initial short-lived enhancement is observed in foF2 records, and it is correlated with the initial positive phase of the geomagnetic storm. The main phase of the geomagnetic storm is correlated with a concomitant foF2 diminution, and this reduction in foF2 may last for a day or longer. It is thought that the initial enhancement in foF2 is a result of electrodynamic forces, while the long-term reduction in foF2 is associated with changes in upper-atmospheric chemistry and modification of thermospheric wind patterns. A key factor in this process is ionospheric heating through dissipation of storm-induced atmospheric gravity waves. This heating effect will cause the thermosphere to expand, and ionospheric loss rates will increase.

Tracking Basics

For automatic target tracking, a sequential procedure must be used to acquire the target and initiate track. The three steps are target detection, target acquisition, and target track.

Target Detection. In order for the received echo signal from the target to be detected by the radar, the receive signal strength in that particular range cell must be stronger than the residual noise in the radar and other interfering signals in that range cell. For a target separated from clutter, the primary interfering source is receiver noise. Although it is desired to declare a target’s presence with high probability, it is also necessary to keep the probability of false alarm (declaring a target detection when no target is present) as low as possible. The two values are tied closely together: for a given signal-to-noise (SNR), lowering the detection threshold to increase the probability of detection threshold also increases the probability of false alarm. Depending upon the target detection criteria, a SNR of 8 to 15 dB is generally required to keep the probability of detection reasonably high, while keeping the probability of false alarms at or below 10- 6. Probability of detection vs. false alarm curves are available in Blake (1) and a number of other sources.

In many cases, the single-pulse SNR may be below the threshold, but the SNR can be improved by integrating a number of pulses. For coherent operation, the SNR improvement is directly proportional to n, the number of pulses coherently integrated. For noncoherent operation, the SNR improvement for small n, is usually near n0 8 in practical radar systems where n < 20. Most real targets are composed of complex reflecting surfaces; the scattering contributions of these separate reflecting surfaces tend to add and subtract vectorially to the overall radar cross section (RCS) of the target. The fluctuations in RCS caused by these surfaces will affect the probability of detection and false alarm. Swerling (2) has derived the probability of detection and false alarm curves for both slowly varying and rapidly varying target RCS fluctuations. For these cases, the required SNR required can be obtained from this set of curves.

Target Acquisition. Target acquisition for tracking can be done either manually or automatically. For manual target acquisition, the operator needs to point the radar antenna (or an angle cursor) on the azimuth angle to the target and designate the desired target range. Alternately, the operator could use a light pen if available to designate the target azimuth angle and range to the tracker. When the particular target is within the acquisition limits of the tracker, the acquisition process can be initiated to lock the tracker up on the target range and azimuth.

For automatic target acquisition, the tracker must have either a designated philosophy for selecting the target for track acquisition, or the tracker must have sufficient capability of tracking all the targets satisfying the track initiation criteria. For example, for a radar altimeter, the track would be initiated on the closest radar returns to the radar. For a scanning surveillance radar, the tracker would need to have sufficient capability to track all the targets satisfying the track criteria.

Ionospheric Response to Solar Flares

Now we shall take note of a special class of effects called sudden ionospheric disturbances (SIDs). These constitute those events that arise as a result of the atmospheric interaction with electromagnetic flux from solar flares. A book by Mitra (33) is an excellent treatise on the ionospheric effects of solar flares.

We recognize that the sun is the ultimate source for a large variety of ionospheric and magnetospheric effects. Fig. 20 exhibits the hierarchy of solar-induced ionospheric effects. There are many types of SID observed; one of the most important is the short-wave fade (SWF), which affects HF communication circuits on the sunlit side of the earth. The source of the enhanced D-region ionization responsible for the SWF is typically an impulse burst of X-ray energy from within an active region on the sun (generally a sunspot). An X-ray flare generates a significant increase in D-layer ionization with a temporal pattern that mimics the flare itself. This results in an increase in the product of the electron density and the collision frequency. It is the growth of this product that accounts for the absorption of HF signals passing through the D region. Flares tend to be more prevalent during the peak in sunspot activity, and the individual-flare duration distribution ranges from a few seconds to roughly an hour.

Intracranial Pressure Monitoring

The brain is surrounded by the skull, and the inner pressure of the skull is almost uniform and is called the intracranial pressure. Normal intracranial pressure is about 10 mm Hg (1.3 kPa), referring to the zero-pressure level in upper cervi­cal spine. However, because of the high stiffness of the skull, a small increase in cranial volume causes a significant in­crease in intracranial pressure. Increases in intracranial pressure are serious, because they cause obstruction of the cerebral blood circulation. Such a situation can occur follow­ing intracranial bleeding, cerebral edema, growth of tumors, infectious lesions, and parasites. Therefore, intracranial pres­sure is measured and continuous monitoring is performed in such patients, especially when increase in intracranial pres­sure is likely to occur.

To estimate intracranial pressure, spinal measurement has been used. Because communication exists between the spinal fluid and the fluid in the ventricles, cerebral pressure can be measured by puncturing the lumbar vertebra. How­ever, such a technique cannot be used for monitoring intra­cranial pressure. To monitor intracranial pressure continu­ously, an invasive method has to be used, except in neonates who have natural openings of the skull, the fontanelles, through which intracranial pressure can be measured nonin- vasively.

Both extracranial measurement using a liquid-filled tube and intracranial measurement using an implantable trans­ducer are possible, and there are many different approaches for each method (26). In extracranial measurement, a liquid – filled tube is introduced into a ventricle, subdural space, or subarachnoidal space via a burr hole made in the skull, as shown in Fig. 8(a), and a pressure transducer is connected to the proximal end of the tube. A catheter-tip pressure trans­ducer can also be used in place of the fluid-filled tube.

Figure 8. Methods of intracranial pressure monitoring: (a) place­ment of a pressure catheter in the subarachnoidal space, (b) im­planting a transducer in a bore hole through the skull, and (c) fonta – nometry.


A pressure transducer that fits into a bore hole made in the skull, as shown in Fig. 8(b), may also be used. In this

configuration, the diaphragm of the pressure transducer can be adjusted to the epidural surface so that the error due to deformation of the dura mater can be minimized. A telemetry system is also attempted in which a small transmitter is as­sembled in the transducer unit and the signal is received by a coil placed on the skin. The elimination of the cable connec­tion is advantageous to avoid infection.

In the neonate, the skull is not completely formed so that openings called the fontanelles exist, and intracranial pres­sure can be monitored noninvasively by placing a transducer on a fontanelle as shown in Fig. 8(c). The diaphragm of the pressure transducer has to be in coplanar alignment with the skin surface so that the tension of the skin tissue does not affect the measurement. When the transducer is adequately fixed, intracranial pressure can be monitored for 24 h or more.

The High-Latitude Ionosphere

From a morphological point of view, the high-latitude region is the most interesting part of the ionosphere. It has been said that the auroral zone and associated circumpolar features, are our windows to the distant magnetosphere, and the presence of visible aurorae has fascinated observers for centuries. The interplanetary magnetic field, which may be traced to its solar origins, has a significant impact on the geomorphology of the high-latitude ionosphere and its dynamics, including magnetic substorm development.

The high-latitude region of the ionosphere is characterized by a hierarchy of phenomena that are largely orchestrated by magnetospheric and interplanetary events (of a corpuscular nature) rather than solar (elec­tromagnetic) flux variations. Hunsucker (28) has examined the salient features and they are depicted in Fig. 17, with particular emphasis on the high latitude trough. In Fig. 18, from Bishop et al. (29), many of the same features are depicted and compared with worldwide features.

Fig. 18. Depiction of various ionospheric features at a given time such that the day-night terminator is passing through the middle of the United States (i. e., « 2300 GMT). [By permission of J. M. Goodman and Kluwer Academic Publishers, Norwell, MA (11), from Bishop et al. (29).]

160° 80° 0° 80° Longitude

The magnetic activity index Kp is generally available and is typically used as the parameter of choice to determine the statistical position of the auroral zone. The concept of the auroral oval was developed by Feldstein and Starkov (30) on the basis of a set of all-sky camera photographs that were obtained during the International Geophysical Year. Other models exist, but the Feldstein picture is found in most models that attempt to include auroral effects in some way. The position of the oval is important, not only as an ionospheric feature in itself, but because it also represents a boundary between the decidedly different geophysical regimes that are poleward of it (the polar cap) and equatorward of it (the midlatitudes). Because the position of the auroral zone varies diurnally as well as with the index Kp, there are some sites that may be characterized by all four regimes at any given time: polar, auroral, trough, and midlatitude. Iceland is such a location.

One of the most fascinating properties of the various circumpolar features is their latitudinal motion as a function of magnetic activity. The ionospheric plasma is best organized in terms of some form of geomagnetic coordinates, but the high-latitude plasma patterns are not fixed in that frame of reference either. The equa – torward boundary of the region of precipitating electrons has been deduced from DMSP satellite instruments, and it takes a form due to Gussenhoven et al. (31):

Fig. 19. (a) Descent of the auroral oval as a function of magnetic activity; (b) position of the auroral oval and its thickness versus the magnetic index Kp; (c) position of the auroral arc formations versus the magnetic activity index Dst. [By permission of J. M. Goodman and Kluwer Academic Publishers, Norwell, MA(11).]

where corrected geomagnetic coordinates are used, L(t) and L0(t) are specified in degrees, and L0(t) is the equatorward boundary of the oval when Kp = 0. It is emphasized that L0(t) and a(t) are functions of time in the magnetic local time (MLT) system.. Both functions are smoothly varying over the diurnal cycle: L0 ranges between 65° at «0100 MLT and «72° at «1700 MLT, and a(t) varies between « -2 at 2400 MLT and
-0.8 at «1500 MLT. Therefore the statistical representation of the oval has its greatest equatorward descent during nocturnal hours. Moreover this equatorward boundary is greatly influenced by magnetic activity. Chubb and Hicks (32) have found that the daytime aurora descends approximately 1.7 degrees per unit Kp, and the nighttime aurora descends at a rate of 1.3 degrees per unit Kp. The auroral oval and thickness are depicted in Fig. 19. Ultimately the auroral arcs, which reside within the auroral oval, are tied to interplanetary phenomena.

Fig. 20. Hierarchy of solar-terrestrial effects. [By permission of J. M. Goodman and Kluwer Academic Publishers, Norwell, MA (11).]

Workers have shown that the magnitude of the southward component of the interplanetary magnetic field is a key factor is the development of so-called geomagnetic substorms, wherein Kp exhibits large enhancements.

The US Air Force prepares daily summaries of an index Q in order to provide a basis for various analyses of the high-latitude ionosphere. The index Q ranges between 0 and 8, with larger values associated with a widening of the oval region and a general increase in intensity of activity within the oval. Moreover, the equatorward boundary of the auroral oval moves to lower latitudes as Q increases. Since Q, viewed as a parameter, defines the shape and location of the auroral zone, it is a convenient index for transmission to communication facilities and forecasting facilities. Its utility is dependent upon timeliness and accuracy. As originally designed by Feldstein, Q defines only a statistical relationship between the oval position and magnetic activity, the latter being parameterized by the planetary index Kp. Nevertheless, the Feldstein oval concept has been shown to have some utility under real-time circumstances. Satellite imagery is used to deduce an effective Q.

Auroral physics is an exceedingly rich and complex subject. Not all phenomena in the high-latitude region are understood, and insufficient data are available to fully characterize even those factors for which a general understanding exists.