Earth's ionosphere is the part of the atmosphere that contains ionized gas that affects radio propagation. Among the several, solar flares are the main sources of the increases in the electron density of the ionosphere. The solar flares are the large scale eruptions on the Sun which releases large amounts of X-ray and EUV flux. The X-ray and EUV flux enhances the free electrons, thus the ionization, in the ionosphere. These disturbances in the ionospheric electron density are called Sudden Ionospheric Disturbances (SID). They can last from a few minutes to several hours. When SIDs occur long distance HF radio communication can be blocked and the blockage may last hours. This is mainly due to the increased level of ionization in the D-layer of the ionosphere extending from 50 to 90 kms. The ionosphere has several distinct ionization layers which reflect the HF radio waves and allow long distance communication and communication with satellites over the globe. It is the electron density level that differentiates these layers and changes the refractive characteristics of the ionosphere. Each ionospheric layer is characterized by a critical frequency which depends on the local variations in the electron density level. The electron density in D layer is usually very low when compared to those in the higher ionospheric layers. It is mainly present in the daytime, but absent during the night. VLF radio wave propagation enhances as the daytime D layer becomes stronger during the solar flare events. During the daytime, VLF signals are reflected from the D layer and the signal is confined between the D-layer. Any change in the quality of the VLF signal level is seen as SID disturbance at the VLF recording station. When a solar flare occurs, the VLF propagation is disturbed and we can detect SIDs by monitoring the signal level of a distant VLF transmitter. In Istanbul Technical University, we record SID variations from seven VLF stations located nearby since June 2011 for the purpose of detecting the solar activity effects on the characteristics of the ionospheric radio wave propagation. One of the VLF transmitters is located in Bafa, Turkey. Observing several VLF stations gives us opportunity to compare the signal levels and helps to detect spatial effect of the solar flares. In addition to the effect of the sunset and sunrise times, solar flare effects, and, to some degree, global lightning activity can also be observed in SID data. The solar flare X-ray flux is recorded by the geosynchronous satellites (GOES series). We use GOES data for solar flare occurrence and study the SID variations associated with the solar flares. The signatures of the solar flare effects on the SID data are seen as the increased signal levels during the daytimes. These times will be compared with those of increased X ray flux measured by GOES spacecraft. The purpose of this study is to determine the typical daily variations of VLF signal strength by examining 3 months of SID monitor data at ITU. In this presentation, we will present our preliminary results based on the ITU SID observations and show examples of typical and extreme SID occurrences and discuss their causes. We also address on the consequences on the radio propagation.