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Sudden Ionospheric Disturbances resulting from an interaction of the Solar Flare radiation with the constituents of the upper atmosphere constitute one of the three major aspects of ground level monitoring of solar flares -the other two being optical observations of flares, and the observations of solar bursts in radio wavelengths. SIDs, therefore, form a major part of flare monitoring programme in many observatories. Unlike the other two, however, the ionospheric effects of flares provide one major additional source of interest - the reaction of the ionospheric plasma to an impulsive ionization. The high atmosphere provides a low pressure laboratory without walls in which a host of reactions occur between electrons, ions and neutral particles. The resulting products and their distributions may bear no resemblance to those of the primary neutral constituents or their direct ionization products. The variations with the time of the day, with season and with solar activity that form the bulk of the ionospheric measurements are too slow to allow any insight into the nature of these ionospheric reactions whose lifetimes are often very short. The relaxation time of the ionospheric ionization is only a few minutes or fraction of a minute in the lower ionosphere and in the E-region and is about 30 min to an hour at 300 km. The flares provide a sudden short impulse comparable to these time scales.

Ionospheric Effects of Solar Flares

1 / Introduction.- 2 / Description of SID Techniques and Observations.- 2.1. The Sudden Enhancement of Atmospherics (SEAs).- 2.2. Sudden Enhancement of Signal Strengths (SES), Sudden Field Anomalies (SFA).- 2.3. Sudden Phase Anomalies (SPA).- 2.4. Short Wave Fadeouts (SWF).- 2.5. Sudden Cosmic Noise Absorption (SCNA).- 2.6. Sudden Increase of fmin (SIF).- 2.7. Flare Effects Observed with Pulsed Transmitters.- 2.8. Sudden Frequency Deviations.- 2.9. Wave Interaction Technique.- 2.10. Partial Reflection Observations.- 2.11. Satellite Radio Beacon Experiment - Changes in Total Electron Content (SITEC).- 2.12. Incoherent-Scatter Technique.- 2.13. Rocket Soundings.- 2.14. A Comparative Evaluation of Electron Density Profile Errors.- 3 / Flare Radiations Responsible for Ionospheric Effects.- 3.1. Quiet-Day Solar Flux in the EUV and X-Ray Regions.- 3.2. Flaretime Enhancements in XUV Fluxes.- 3.3. X-Ray Spectral Development.- 3.3.1. The Equivalent Spectrum.- 3.3.2. The Composite Spectrum.- 3.3.3. Spectral Development Based on Free-Free (Bremsstrahlung) and Radiative Recombination Transitions.- 3.4. Photoelectrons During Flares.- 4 / SID Phenomenology.- 4.1. Occurrence of SIDs with X-Ray, EUV Flares and Microwave Bursts.- 4.1.1. D-Region SIDs and X-Ray Fluxes and Microwave Bursts.- 4.1.2. F-Region SIDs and EUV Flux.- 4.2. Threshold X-Ray Flux for the Production of an SID.- 4.3. Level of Solar Radio Flux in the 3000 MHz Region and Its Relation to the Occurrence of Sudden Ionospheric Disturbances.- 4.4. The Size Correlation.- 4.5. Time of Growth and Relaxation Time.- 4.5.1. SEA Types and Cosmic Rays.- 4.6. SDA Characteristics and Implications.- 4.7. Examinations of Complete Time Profiles of X-Ray Flares, Radio Noise Bursts and SID Effects.- 5 / Some Simple Analysis Techniques.- 5.1. Photoionization Rates and Equations of Continuity.- 5.2. Use of Relaxation Time.- 5.3. Decay Curve Analysis.- 5.3.1. Principle of the Decay Curve Analysis.- 5.3.2. Decay Curves Analysis for Absorption Events.- 5.3.3. Decay Curves of SPAs.- 5.3.4. Decay Curves of Magnetic Crochets.- 5.4. Simultaneous Correlation of ?? and ? A.- 5.5. Zenith Angle Dependence of SIDs.- 6 / Some Outstanding Solar Flare Events.- 6.1. Ionospheric Effects of the Flare of July 7, 1966.- 6.2. Ionospheric Effects of the Solar Flares of May, 1967.- 6.3. Ionospheric Effects of the Solar Flare of July 8, 1968.- 6.4. Ionospheric Effects of August 1972 Flare Events.- 7 / Development of Electron Density Profiles From Synoptic SIDs.- 7.1. Profiles from SCNA Measurements.- 7.2. Profile Determination from Simultaneous Use of SCNA and SPA Measurements.- 7.3. Profiles from Phase and Amplitude Measurements at VLF.- 8 / D-Region Ionization Profiles and Loss Rates During Flares.- 8.1. Flare Ionization Profiles.- 8.2. Loss Rate During Flares.- 8.2.1. From Quantitative Relationship of Flare X-Rays to SIDs.- 8.2.2. Average Profiles for Weak, Moderate and Strong Flare Conditions.- 9 / SIDs as a Tool for the Study of Aeronomy and Ion Chemistry.- 9.1. Photoionization Rates During Solar Flares.- 9.2. Minor Neutral Constituents in the Mesosphere.- 9.2.1. Nitric Oxide in Mesosphere.- 9.2.2. Ozone in Mesosphere.- 9.2.3. Water Vapour and Other Hydrogen Compounds.- 9.3. Ion Composition.- 9.3.1. Ion Composition in the D-Region.- 9.3.2. Ion Composition in E- and F-Regions.- 9.3.3. Ion Chemistry.- 9.3.3.1. Important Reaction Rates.- 9.3.3.2. Positive Ion Schemes.- 9.4. Information on Nitric Oxide from Simultaneous Measurements of Electron Density and Solar X-Rays During Flares.- 9.5. Chemistry of D-Region During Flares.- 9.5.1. Chemistry at 80 km.- 9.5.2. Negative Ion Regions (70 km and below).- 9.5.3. Models of Flaretime Ionization Profiles.- 9.6. F-Region During Flares.- 9.6.1. Ionization Changes.- 9.6.2. Effective Loss Rate.- 10 / SIDs as Flare Radiation Monitoring Techniques.- 10.1. Information on the X-Ray Region.- 10.1.1. VLF Observations.- 10.1.2. HF Absorption Phenomena Including SCNA Observations.- 10.2. Information on EUV Flux from F-Region SIDs.- 10.2.1. EUV from Ionograms During Flares.- 10.2.2. SFDs as Ultraviolet Monitor.- 10.2.3. XUV Flux Determinations from Incoherent Scatter Observations.- 10.2.4. Combined EUV Information from F-Region SIDs.- 11 / Polar Cap Absorption Events.- 11.1. Radio Measurement Techniques.- 11.1.1. Use of Riometers.- 11.1.2. Use of Ionosondes.- 11.1.3. Partial Reflections.- 11.1.4. LF and VLF Observation.- 11.1.5. Oblique HF Propagation.- 11.1.6. VHF Forward Scatter.- 11.2. PCA Morphology.- 11.2.1. Classification of PCA Events.- 11.2.2. Microwave-Millimeter Wave Solar Radio Bursts and Proton Flux.- 11.2.3. Twilight Effect.- 11.2.4. Solar Cycle Variations in the PCAs.- 11.3. November 1969 PCA Events.- 11.4. Principal Aeronomic Features of the Lower Ionosphere During a Major PCA.- 11.5. Chemistry of the Lower Ionosphere During PCAs.- References.- Special Data Volumes, Books and Conference Proceedings.- Index of Names.- Index of Subjects.